[Gregs71]

You didn't say if the truck has overdrive or not and whether its an auto or stick.

What you want to do is probably possible ( 5.3 TBI with stock tranny / TC), the least efficient, but the easiest to do.

The next efficient combo is the full fuel injected (means swapping wiring harness and computer) with stock tranny / TC.

If you have a 3 speed tranny, the most efficient combo is the 5.3 fuel injected ( not TBI) with overdrive. If the 4 speed is a modern 4 speed (4l60/4l80) then you have the added difficulty of mating it to the transfer case. If your running the 700r4, then your mating the 5.3 to the 700r4. If its a manual then and you add overdrive, your back to having difficulties mating a new OD manual tranny to your 5.3 and stock TC.

I have been kicking a round a similar idea. I'd like to hook up a 6.0 to my th400, more for reliability then efficiency though.