Wont start and it backfires

My truck has had a sputter for a long while so I set about trying to fix it. After hundreds of dollars I finally got it right and it ran like a brand new one. Ran great about a week then one morning about half mile from home it lost power, backfired through the tail pipe and shut off. After getting it towed home, I cannot for the life of me figure out what is wrong. When cranking it speeds up and slows down like the timing is jumping around, then backfires through the tail pipe. It has spark, it has fuel pressure (60 psi), it gives me no codes. Here are all that has been changed. Timing set, crankshaft position sensor, plugs, plug wires, distributor, spider injector harness with new injectors, upper and lower intake gaskets, head gaskets, and had both heads reworked. It's in time, with number one TDC the distributor is pointing at number one. Bottom end marks are lined up properly as well. Anyone have an ideas?