My truck back fires out the exhaust after cranking it over for a minute or so sometimes backfires through the carburetor.  The truck was running mint but I mustve revved it to high.  The timing is perfectly alligned on the motor because that what my first suspicion was.  Ive read it could be the primary coil but anyone that knows anything please let me know.  

Next step may be to pull the valve covers and make sure everything is still  moving right.  You may have cracked a rocker arm or bent a pushrod or something. You may have even wiped a cam lobe.  What type of carb do you have?  If Holley, is it possible you blew a power valve?  I do not know much about Holleys. Will it start and run despite the backfiring?  If it doesn't even start you may have broke your timing chain, though I think that would cause a racket under the cover.