Muffler Replace Feeling Dumb

[papasink]

Okay so I have to admit, I’m feeling pretty stupid right now. I have the 19 RST 5.3L 8 speed and I’m looking to replace my muffler. I am going with the Magnaflow but they have 3 length options. My existing pipe is 2.5” OD. It measures 21” at the muffler itself. 
 

Here are my questions: 

 

Do I order the 2.5” muffler or do I need a bigger size since my pipe is 2.5. Again, feeling dumb because I never did exhaust myself. Also do I clamp or weld on place. I plan on cutting the pipe to have a direct fit without any add in pieces of pipe. 

 

Is a smaller length muffler better for flow and sound or doesn’t that matter. 
 

Thanks for the help. I have the screens covered and it sounds good but I like the new Magnaflow sound. 

[DemonWorks]

Muffler specs on the new one need to be 2.5" ID if you plan on slipping it over the existing pipe.

 

Weld in seals the best

 

Clamp works

 

If you are going to a muffler shop to install, they can make necessary adjustments.

 

Typically a shorter muffler is louder. Magnaflow design is straight through with packing, so the flow should be really close to the same.

 

I do know magnaflow was working on direct replacement mufflers for our trucks, not sure if that has been completed though.

[f8l vnm]

Why not do a muffler delete ? Cheap/more sound and easily can go back to stock if you dont like it 

[AJMBLAZER]

Pretty sure the exhaust is 2.75".  Mine is and GM's used that width for their small block V8's since the late 90's.