[Boog]

There is a lot of speculation here about your engine specs. You won't really know what's in it unless you tear into it and see for your self what pistons, cam lift, duration etc it really has. It sounds like a fun engine for off road adventures but maybe not so good for the street.
So far no one has asked just what shape your brake system is in. Have you rebuilt the complete system or just know it doesn't feel right? If the truck once stopped good but now doesn't I would pull the wheels and inspect the whole system. Low vacumn will diminish power brake performance. If I recall correctly the factory system needs a minimum of 9 or 10" of vacumn to operate right. There could be a small vacumn leak in the brake booster reducing it's effectiveness. If it is an original brake booster it would be suspect due to age. There is a coil spring inside the booster that can break and rub a small hole in the diaphram bleeding off vacumn. I've had that very thing happen on an 82 pkup. That truck had a stock exhaust and I could not hear the vacumn leak. You could remove the vacumn line from engine to booster and plug it and see if the vacumn signal goes up at the engine. If so it can indicate a vacumn leak in the booster.
A vacumn reserve can from the salvage yard could be a good tool to narrow the cause to either just a low engine vacumn or worn/weak vehicle brake system.