I have a 95 camaro that i put a 94 motor in. when you try and crank the motor it turns over then it will backfire up through the intake i know the timing has not been messed with because the motor was running when pulled about 6 months ago.

 

Crank Position Sensor?

 

Definitely has something to do with the ignition system. I had a bad coil one time and it did the same after cranking it for a little while. Make sure all wires are on the right coils and right plugs.