Rough start and backfire

ok… I have a 92 Full size Blazer, 5.7 350. My son and I are rebuilding it and have replaced the motor with a crate engine from Summit. Obviously new wires, plugs, distributor and cap. Rebuilt injection system and new injectors.

ISSUE: When we turn it over it acts like it wants to start REALLY ROUGH and then back fires through throttle body. I know it has to be a timing issue… Could I have missed something when installing distributor? Is the timing just that far off? We put distributor on before we stabbed motor, set at top dead center on #1, i turned the cam gear to install distributor to line up number 1 cylinder (or i felt really close), I have checked all wires and tested spugs at distributor and have a spark on all. At a loss, any ideas would be appreciated!

Thanks, Roy