[Bruce Edwards]

One question I do not see and I did not read in your post. Did the engine run before and then die? Now when you turn it over it backfires out the tail pipe? If so how many miles on the engine? Over 100,000 ? If so sounds like the timing set has gone bad and the cam timing is now out of time.
Easy check is to run a compression test. Also as someone else said spin the engie to TDC mark on balancer and pull the cap. Is the rotor pointing at #1? You can check for number one compression by pulling #1 spark plug and checking for compression as you turn the engine BY HAND not with the starter. If the rotor is not pointing to #1 the timing has jumped.