Further to tracking the poor mileage, I'd determine the exact consumption prior to tearing my hair out. Connect a gallon can direct to the carburetor (not the fuel pump) and drive the truck until it coughs out; re-fill, and drive back the same way. Note the mileage. That's a start point. Next, pull the plugs and examine the porcelain. Plugs are the 'eyes' of an engine. They will very often reveal problems associated with particular malfunction. Check also the tail pipe residue - heavily carboned, damp, points to unburnt gas. Another step is to re-check fuel pump pressure. Old, or new, pumps are notorious gas wasters if not operating within specs; and a leaky diaphram can really affect consumption. Then too, is the carburetor itself. From float level, to needle seat, to dirt, many little things plague carb function. But that's part of the fun(?) to be had with these vehicles - chasing down the gremlins and tweaking every part to wring best performance. Plus, when everything is perking, guess who gets the bragging rights at the next mileage swapping tales? :o)
Laurence
