No start but did backfire

I replaced all the gaskets from heads up. May have disturbed timing by putting distributor in at a different position than it came out. Cranked nice seemed like no fire. Went through the process of TDC of cylinder number one compression stroke and distributor is seated correctly, I believe. Cranked it for about five seconds and as I stopped, there was a backfire. Looking for suggestions what to do next and what is causing the backfire? A couple months before I did the gaskets, my muffler literally blew into pieces. Do not know why! Truck has ran since the muffler explosion and was running when I parked it to do the gaskets.