94 jimmy starts and runs for a few min and dies

hi all have a 94 jimmy with a 4.3 vortech the guy was driveing it andit back fired and then would not start they brughtit in and i have changed the plugs wires cap and rotor timeing chain and sprokets balance gear and distributor drive gear it i all lined up on number 1 top dead center i have 55 psi fuel pressure so i really dontknow where to go with this any more any help would be greatful

Fuel pressure should be 62-66 PSI.