Why wont my truck charge? the alternator is new and the wiring is new?

I currently had to rewire my truck and now it wont charge. i know it isnt the wiring harness because i was able to isolate the engine from the rest of the truck and it still wont charge. it is a one wire gm alternator on a 1989 chevy k1500. the motor is from a 74 nova and is carburated. the wiring harness is a ez wire harness. i am only able to get 12 volts and it seems to just run off the battery 90 percent of the time.