1988 GMC S-15 Pickup 2.5Liter "putting, spitting, backfir

I have replaced the vac hoses, EGR valve, Fuel pump, Carb Gaskets, Fuel Injector, Air Filter, Tried new carb, Oxigen Sensor, Thermostat, Thermostat Sensor. This truck idles fine and when you give it gas it can not take it? It puts, spits, backfires through the fuel injected carb. It has 60,000 miles on it. I've dropped the fuel tank and drained it thinking it could be water, I've even replaced the fuel pump while I was at it. I'm thinking it can't be timing with 60,000 miles on it. I've tried to check the timming but can't turn the distributor because it seems stuck. I've also replaced the distributor cap, rotor button and spark plugs. Still runs the same. The exhaust is totally messed up, the Catalytic Converter has a hole in it and has no more exhaust so it's cut off from the Cat back, no muffler or anything. I'm thinking it also needs an O-Ring put in it at the Manafold, could that be the issue, because it has no back pressure and it's only a 2.5 liter. It seems like a air leak somewhere and I can't find it. It putts and spits but I have to really put my foot in it and sometimes it clears out, but not really because it will keep putting, and spitting and backfireing. If it was timming it wouldn't idle so that theroy goes out the door. It idles fine, until I give it gas. Please help me. I paid $50 bucks for tihs truck and I know I've put alot of money into already. It worth saving.