Why does my Chevrolet G20 van act like it does not want to pull?

I have a 1990 Chevrolet G20 van. My dad has rebuilt the motor, changed the transmission fluid and filter, put new plugs and wires, put a new radiator on and several other things. It acts like it does not want to pull hills and stuff. We can not figure out what might be wrong.|||the catalytic converter is probably plugged|||the catalytic converter may be stopped up on it or it may not be getting enough gas to run good enough on,check your fuel pressure,and also re-check the timing,timing is really important on those ,if its off just a little it wont run right good luck on it.