Hesitation and backfire under load

Hello me and my grandpa just swapped a good running 350 for a 1970's truck into my r-10. When at idle in park it runs fine and you can accelerate and it won’t hesitate. But will occasionally backfire. Timing is dead set at zero with the timing advance unplugged. Injectors are for a 350. Has new coil and EGR valves. New map sensor. O2 sensor. Don’t know what else to try. Any help would be appreciated.

But it does not hesitate or anything nearly as bad if it’s cold. But once it gets warm then it starts spitting and sputtering. I can take a video and post it here if you would like.

Thank you