1990 chevy lumina 3.1 MPFI and will not start?

The car will start if you hold the gas have way down and hold it and you have to keep pumping the gas to keep it runing sometimes. I replaced the head gaskets and thought that maybe my problem but its not. I also used it as a snow plow for two days ( snow was packed in the engin pay up to the exhaust and i had to jack it up and put a heater under to defrost it). If any one has any answers i thank you very very very much.