360 Won't Start!!

Ok guys... I just got done getting my 360 all put back together. I tore it down to put some higher compression pistons in it about 3 weeks ago. So last weekend I got it all put back together and went to start it and.... nothin. It just cranked and cranked. So I checked fuel at the carb which I had so I figured it must be spark... well after I checked a few things I knew I had spark too. So, my question is... it must be my timing right?? Now I played with my initial timing a little and the best I could get was some gas spitting up through the carb like a backfire. Now this is my first re-build like this so can you guys help?? What's going on? Is there anything internally that I could have screwed up? Or could it be a vacuum leak? Any help would be great guys thanks!