Engine trouble

[racerkyle]

I have a 1996 GMC Z-71 with a new crate engine installed. All plug wires, plugs, distributer etc.. is new. A P0300 code is popping up and it causes the engine to miss, but when I clear my computer, the engine runs great then I drive it 10 or 15 miles and the code pops back up. It says it's a Multiple/Random Engine Misfire detected. I don't know what to do. The truck is scking gas really bad. And I smell gas a little when I get out of the truck after drving. Some people are telling me it is the fuel injection stuff, may have to be replaced. Also after I get my truck back from getting the engine in, the transmission will not go into lock-up some times. Like half the time.The torque convertor is maybe 4 months old. Took it to a tranny man and he told me there is nothing wrong with the tranny. Maybe electrical thing. Nothing major, just hard to find. What do I do. Help! Thanks!