68 lemans 400 engine cuts out

I have a 68 lemans with a 400. ive got intake, headers, msd ignition coil and distributer, exhaust, th400 trans with heavy shift kit. my problem is when im diving down the road the engine cuts out, no sputer it just dies. i just changed the alternator and charged the battery. when i restarted the car 1 of 5 times it backfired. i did make t home but not sure whats causing the engine to cut out. I have not checked the timing yet but ill be putting my timing gun on it. any input would help. thanks