GMC Repair: 84 GMC pickup, gmc pickup, timeing

Question
I have a 84 Gmc 2 wheel drive with a 305 engine. Will sometimes backfire going down hiway and after about 15 miles when engine is good and warmed up stalls with hard restarting, have been lucky restarting after couple minutes cranking. New plugs and fuel filter, fuel pump have been installed and also replaced one plug wire that was shorting out. Alos seems if I pump the footfeet alot it seems to help with the resarting.

Answer
hello brad
did you change the dis. cap and rotor .  This will cause it to back fire and stall if it as a hair line crack you can not see it with the eyes .

If you did then check the timeing could be off.

and if not then check timeing chain and gears
pete