need help with a 95 blazer

I have a 95 blazer 4.3 vortec. Can you give ideas on what I can try it fires but wont start, it wants to. Its getting gas. It like backfires pops whatever where the plenum cover goes. I have stuck a lot of money replaced fuel pressure reg, plugs and wires, spider. fuel lines. Thing is I bought it for my son and after a couple weeks the head gasket needed to be replace. Well he was alittle impatient and had way to many people working on it. Now I got it back in a mess. Some ideas would be greatly appreciated thaks