I have a 86 with 302/aod/efi etc etc. The speed density systems on these i would say are pretty reliable but just not the best for making power and tuneability. 85.5 efi was an option and 86s were efi. I'd first check IAC and gasket, clean it out good with some throttle body cleaner or MAF cleaner. Check basics, plugs n wires, leaks, open or damaged wires/connectors. Wanting more power for the 302 and not tearing the block down is a hard thing. Redo the exhaust with some good mufflers or glasspacks for better flow and of course noise
. Ive seen homemade intakes guys have made for their trucks and seems to work pretty good. Intake manifold, cam, and port heads if you wanted to, A 351 would overall be a better platfrom. Pretty much anything 80 to 96 are swappable on these trucks. Is it a 3 or 4 speed auto? The aod 4 speeds are weak in overdrive but can be built solid, many drag cars use aods and handle alot of power.