If you go 4.0 you need new welded motor mounts probably a different tranny Lilly a new transfer case input gear and a PCM but you ca adapt your harness easily but do need different exhaust components
4.3 isn't terrible on fuel at all. I tuned my buddies Sonoma and he gets just over 700km to a tank in which he regularly puts about 60 litres in. If you use all the calculators etc, that is 27-28mpg which is not terrible by any standard.
That being said, the 4.3L isn't the best of motors to work with. When you ask which motor would be best, what is your end goal that you are trying to achieve?
If you want to go Chevy, look into the 5.3's. You'll still get decent mpg and those things can be made into a power house. When guys tank LS motors most go to a 5.3...cheap, tons of aftermarket, easy to build, strong as shit, very easy to make big power with
Now that being said, as stated already, I think that swapping a non jeep motor into these vehicles will be harder than swapping in a jeep motor and likely more expensive to. I personally wouldn't waste time or resources swapping in a 4.0L motor though either.
Depending on the year (assuming it will not be a 2014 used in the Silverado) you would have 30-50 more ft/lb of peak torque with the 4.3 over the 4.0, but it seems like a lot of work for that much/little. Why not go straight to a 350 V8? A V8 swap is probably covered a lot more on the internet too.
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