My GTP won't start please HELP!!!

I have a 1997 Grand Prix GTP.
I Just replaced the block, I had it running for a few minutes and then it did a loud backfire.
Now it will not start at all.
It will crank.
It has spark and it's getting fuel but will not fire.
I'm not sure what else to do!!

PLEASE HELP!

welcome to AF

Try taking the upstream o2 out and see if it will start. If it does then you may have a bad/broken catalytic converter or other exhaust system blockage.
Do you know what caused the first engine to go?

I wold do a compression test just to verify the timing chain hasnt skipped or broke.

Not sure what caused it to blow, Had a crack in the block and sideways piston. Along with broken rods.

Thanks for all your help though I'm going to look into these things.