Swap 4.3L for 5.3L

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Does anyone have any Idea how difficult this would be, I have a 2003 Sierra 4x4 regular cab with an automatic, I bought the 4.3 V6 for mileage and have been a little dissapointed. I see complete 5.3L engines complete with the PCM and harness for sale for $2000 to $2500 and sometimes less listed on E-Bay and sometimes locally. One issue that I know of is the drive by wire on the V8 as opposed to a throttle cable on the V6, but if that is the only issue, it seems doable.

 

Any advice would be greatly appreciated.

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