Could you have slipped off mark when you were putting it back together? valensage

First of all, thanks for all the answers. I probably didnt give you enough info on things. Here goes: I built a 454 chev. It has tunnel ram, 2 4's, 11:1 compression and other goodies. It has a unilite mallory ign (all new) all new timing chain and accessories, short primary headers. It back fires through the exhaust. Someone told me to unhook the vacum from the dist and plug it. I did and no good, still does it. I have tried timing it from 0 to 30 advance and still does no good. Also I have it running on 108 octane fuel. And no, I have no smog pump. Thanks,, Mike