Wont Fire.... Need any input....

[gavinmosher]

Okay guys i have a 84 chevy swb with a 400 sbc out of like a late 70's c10..... it ran.... i wont say good but it ran about a week ago... well i changed the spark plugs, wires, and tightened the loose rocker it had.... i got it to fire up one time after that and then i tried starting it again and it backfired hard up through the carb and now will not fire to save my life.... it will crank all day, has spark.... not sure if its a strong enough spark but has it on each plug and has fuel..... i rolled it around to what i thought was top dead center and pulled the cap and it was pointed at the number 1 plug wire.... so i didnt think it jumped timing but every once in awhile it will thump on every cylinder like its barley firing and then when you let off the key it will blubber (no other descriptive word sorry) to a stop..... any ideas??? And i do have electronic ignition but the guy at autozone gave me plug wires from a 78 with points... so im at a loss right now guys