"I added an Edelbrock SP2P manifold (designed more for economy than max hp), a Carter 625cfm 4bbl carb with vacuum secondary" Todd is correct. Get a 4 bbl carburetor with a spread bore, another words small primary and large secondary that is vacuum operated. advance your timing until it pings but no detonation. Get an intake manafold from a junk 302 with the right carb. and use the original air cleaner with the preheater. it will put warm air into the carb. I got similar results with a 1980 Chevy van, engine rebuilt with a towing cam and transmission with a shift kit set for towing. it went from 14 mpg to 16 and more horsepower. I also removed the pollution controls and used an O2 sensor and gauge to tune the carburetor. Do not waste your time with HHO. it is worthless. I tried on a mercury sable with two trolling motor batteries and it was NO DIFFERENCE. if you come across $15K you can buy this,
AC Electric Drive Systems and the batteries, etc. and have a machinist make an adapter for the front of your engine (double roller 50# chain?) to the ac-50 motor. (I used the ac-20 on my electric motorcycle) A tax deduction and when you junk the truck years down the road just unbolt the electrics and use for another vehicle.