4g64 swap into my 86 G54 pickup

[FMS88]

Is the 4G64 an improvement over the older G54b carburetor motor? Better power? Better gas mileage? Longevity? Even if you may not have personal experience doing this swap I'd appreciate any opinions or ideas. Thanks in advance! Bill

The stock 4G64 for the trucks has roughly ten more horsepower and very slightly less torque. With your mods to the G54b, there's probably little if any difference in HP. The smaller displacement and FI of the 4G64 should in theory yield slightly better gas mileage, but the difference may not be that great. The 4wd 5spds from 84-91+ all have the same gear ratios, but final drive ratios varied across years and models. A lower rear end ratio improves economy, but results in slower acceleration.
Why was the bed and frame chopped off the '91? Was it in a collision? If someone cut up a serviceable vehicle, I'd be suspicious of the state of the engine or transmission. Very thorough testing seems in order before considering either for a swap.

Found the EPA ratings:
91 4wd w/2.4 19/22 https://afdc.energy.gov/files/pdfs/1991_feg.pdf
86 4wd w/2.6 19/23 https://afdc.energy.gov/files/pdfs/1986_feg.pdf