77 custom not starting HELP
 

I have 1977 F150 custom with a 302 v8, i was driving and all the sudden it backfired and stalled, it was running great right before this happened. I replaced the sparkplugs, plug wires ,coil, cap and rotor, nothing so far. I checked fuel pump,fuel filter, iam getting fuel to carburator and iam getting spark. While iam turning the key to turn it over it seems like its trying to run ,even when iam pumping the gas its like its trying to run then i let go of key it stops everything, but if i continue holding the key trying to turn it over it sounds like when a car is running and you try turn it on again while running. Iam lost please help!!!!!!!

Sounds like it might be internal. Probably jumped timing. To check that...pull the plug on cylinder 1 and put a compression tester on it, manually turn the crank until #1 is TDC. Then check your distributor make certain that the rotor is on #1 position for the cap/wire. I would also pull the valve cover and check to make certain that both the exhaust and intake valves are closed. Adjust if off and try starting again. If you get it running, check your timing and adjust.