Question

i have a 68 chevy pickup 4x4, 4 speed tranny and a 350 motor. i just put the motor in a mounth ago. when i would drive it every now and then when i would come to a stop it would just die. i drove it out to my buddies house it was fine. a guy told me the timing was off becasue when it was hot id crank on it for a bit and when it did start it would blow black smoke. so when i got home i played with the timing now when i drive it it will quit when i hit 55 mph but if i let off right away it all keep running. i put a new fuel filter in and cleaned up the terminals on the cap and cleaned the rotor with emory cloth. it got a bran new fuel pump on it so im pretty sure it aint that. It does have the point style distributor. do u think becasue the timing is off far enough it doin that or should i replace cap, rotor, plugs and plug wires?