1997 GMC Jimmy will not start

Had to replace crank shaft and main bearings. Engine now back together. Timing marks at 6 & 12, engine rolls over but back fires thru intake, have tried changing position of the rotor in the distributor, but still will not start. There is fuel, spark
I know there are many things that could be wrong, but I am having difficulty trying to think of them all.
My son is in the middle of a rebuild of a another GM engine and he told me that the timing marks on his engine are at 12 & 12, my manual says 6 & 12, who is right?