1974 Dodge Dart Magnum Swap

Well, I believe most every rotating issue has been looked at. Given that it is a used engine I have not seen where you did any basics to check the operation of the engine.
Like: Pull the plugs to confirm the are the same color, New plug wires, New plugs, Dist cap. I bought 3 dist caps before I had one that was NOT cracked(new ones). Distributor shaft, is it straight? Rotor button? Compression Check, MOST important. Cracked heads? 5.9's are famous for cracked heads. Vacuum leaks. The intake is critical when it comes to the sequence and the small amount it takes to torque it. Cam lift, (worn lobe) pushrods, rockers, lifters. Blown head gasket, leaking from one cylinder to another.
The Scamp I bought with the 5.9 had a slight miss. And it would light up the tires. I never checked what the issue was since that 5.9 was not going to be used.
Hope for the best, expect the worst.