Will NOT run right!!!!!!!!!

The 74 model does have a seatbelt interlock circuit that prevents start. Its very easily defeated. Just clip the 2 yellow wires and connect them together. I think one of them has a black tracer. This circuit only interrupts the start relay anyway and cant effect run.
I would get a temp gauge and find out just how hot it runs.
Too many times a new engine gets all the money and the radiator gets ignored.
If the torque converter is pushing on the thrust bearing, it will run hotter and stall too.
Just my guesses.