Keep the straight six or get a 450 big block?

[Dirtyharry]

I have a '67 gmc currently with a 292 straight six with four on the floor. I am thinking about rebuilding the straight six and keeping it, i am unsure about it. I can get my hands on a big block 450 automatic, without a transmission though. It would cost $500 to rebuild the straight six due to a broken piston seal. Then with the big block i would probably have to or want to rebuild it aswell. I am also unaware of what the best kind of transmission to get for it and the price of it. I won't really be abusing it too much. I just want to know what would be the best path as far as money goes I'm honestly fine with either or. Any suggestions.

[2018GMC]

Are you restoring the truck? If so keep it original and rebuild the straight 6. If it's just a project truck or driver, put a 350 or 454 in it! As far as automatics go, a turbo hydramatic 350 3 speed auto was a factory option. You could also put a 700R4 4 speed automatic in that has overdrive with minimal mods to crossmember, ETC.

[Dirtyharry]

Thanks. I am in between restoring it or just beefing it up. The truck was originally from Montana for farm use. Body work is needed, I don't know if the engine is original I have to look up the codes.

[grampadirt]

Keep in mind the 454 is much heavier and has a lot more torque.Drive the truck easily and it may stay intact,drive it hard and things are gonna break,suspension & brakes should be upgraded.I believe the frame for a big block truck is heavier as well.

[Carder09]

Go modern and throw a 7.0L motor in the truck.

[Friz2]

The truck uses the same frame and brakes no matter what engine comes out of the factory. Front springs are probably a little stiffer for the extra weight of the V8.

With a 6 cylinder, you probably have a 4.10 or 3.90 rear gear. That 454 is going to turn a lot of rpms, but it will get there quickly.

Fortunately, there are literally hundreds of those rear ends lying around with just about every gear ratio imaginable.

Back in the late '80s, I had a 1965 C10 with a 4:10 rear gear. I put a 350/350 combo in it and found a rear end out of an early 70s truck with aa 3:08 gear. They were both trailing arm rear suspensions, so the new rear end assembly bolted right in. I did have to fab a new sway bar due to a different mounting point, but that was pretty easy.

If you can't find another trailing arm rear end, the center section can be swapped easily, you just have to shim the gear lash between the ring and pinion.

The 4 wheel drum brakes suck.

I put a front clip from a mid '70s Chevy van in mine. It bolted right in with disc brakes and power steering.

GM vehicles are so easy. There is really very little that is different in models and years, providing you stick with the same general size of vehicle.

 

 

On the other hand, if you rebuild the 6, you are going to want more power. Chevy I-6 motors were never known for power, but they are very dependable.

 

You have a fun project ahead of you if you go with the big motor. If you're willing to search and work, it can be done very cheaply.

[KARNUT]

Mild built 350, overdrive trans, you'll have the power and decent fuel mileage.

 

 

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[doverarjim]

Back in the day when I was racing a GM 6 cyl we always bought small block parts so we could have extras of things like valves, lifters, rocker arms, because they were the same and if you every broke one the singles were more expensive. Also a big block will not turn a lot of RPM compared to a small block. BB makes torque from about 2,000 rpm to about 7,500. A sb will go from about 3,000 to 9,500 rpm fast but not give as much torque.

If you build the 6 cyl there was at one time some pretty trick parts you could get for them we had a 64 Nova drag car that ran low 11's with a power glide, we held some national records with that car many years ago. Just my $0.02

[Dirtyharry]

The truck uses the same frame and brakes no matter what engine comes out of the factory. Front springs are probably a little stiffer for the extra weight of the V8.

With a 6 cylinder, you probably have a 4.10 or 3.90 rear gear. That 454 is going to turn a lot of rpms, but it will get there quickly.

Fortunately, there are literally hundreds of those rear ends lying around with just about every gear ratio imaginable.

Back in the late '80s, I had a 1965 C10 with a 4:10 rear gear. I put a 350/350 combo in it and found a rear end out of an early 70s truck with aa 3:08 gear. They were both trailing arm rear suspensions, so the new rear end assembly bolted right in. I did have to fab a new sway bar due to a different mounting point, but that was pretty easy.

If you can't find another trailing arm rear end, the center section can be swapped easily, you just have to shim the gear lash between the ring and pinion.

The 4 wheel drum brakes suck.

I put a front clip from a mid '70s Chevy van in mine. It bolted right in with disc brakes and power steering.

GM vehicles are so easy. There is really very little that is different in models and years, providing you stick with the same general size of vehicle.

 

 

On the other hand, if you rebuild the 6, you are going to want more power. Chevy I-6 motors were never known for power, but they are very dependable.

 

You have a fun project ahead of you if you go with the big motor. If you're willing to search and work, it can be done very cheaply.

Thanks i haven't checked these forums in awhile. A buddy of mine gave a SB350 i'm just going to use the standard transmission that came with the straight six. The cylinder head was cracked, the second piston is destroyed. Something was bouncing around. So I was decisive of whether i should rebuild or swap. When we took the tranny out, turns out the pressure plate and and clutch are new. Might as well use it right? whats the best rear end to use i mean the stock one would work right? I was thinking of disc brake conversion. and sooner or later going ls.

[SouthernSilveradoGuy85]

If it's going to be a show truck where you enter shows where having matching #s are required or helps then go with the 6, but if not then do what you want to with it. I personally would but any fuel injected V8 in it. Or thinking out of the box, Duramax lol.

[Friz2]

Thanks i haven't checked these forums in awhile. A buddy of mine gave a SB350 i'm just going to use the standard transmission that came with the straight six. The cylinder head was cracked, the second piston is destroyed. Something was bouncing around. So I was decisive of whether i should rebuild or swap. When we took the tranny out, turns out the pressure plate and and clutch are new. Might as well use it right? whats the best rear end to use i mean the stock one would work right? I was thinking of disc brake conversion. and sooner or later going ls.

I'm not here very often, either, so if we keep up this back and forth, it will be a long thread...

If the 6 has parts tumbling around inside, there is likely more damage than you will be able to see. With all the running motors available, I'd scrap it. It would be a shame to spend money rebuilding to find out later that you have a crack in the block, crank, or main journal.

Anyhow, I'm not sure if the I6 and V8 clutch are the same. I haven't spent much time with GM I6 motors. The manual tranny will be fine (3 speed?). The rear end will work fine, but as I said before, you are going to run some high RPMs.

[grampadirt]

Thanks. I am in between restoring it or just beefing it up. The truck was originally from Montana for farm use. Body work is needed, I don't know if the engine is original I have to look up the codes.

I go to LMC-Truck.com when I have a question like yours.Check out this page taken from their '67-'72 catalogue.Check your VIN against this page,should help identify your truck & engine.

 

[Cowpie]

Any pickup like that, I am firmly in the rebuild the straight 6. the 292 was a great engine. And all the work you do to it, the value will be higher with factory original engine. I have always been fond of inline engines. Hard to kill them and good low end torque.

[STPETEBLUE]

Curious as to what you ended up doing. I am with the "keep it original" crowd.