I don't think the seat belt warning system will cause a no crank (only warning sound and light), 1st thing i would do is make sure shift linkage is adjusted correctly, next make sure you have a solid ground from engine to body.
I know on my 74 if the floor/seat harness was plugged in, the engine wouldn't crank at all unless the seatbelt was hooked in. I didn't think this was on cars until 74 thought. But anyways, if he has plugs going to his seats and seat belts, simply unplug them to kill the safety feature.
