440 problems ,new motor

The friction of the load from the bolt squeezing the gear onto the cam is what turns the cam.
The dowel pin is soft and is just there to align the gear to the cam.
If the bolt gets loose, the dowel will just shear right off.

One important component in the single bolt cam set up is the washer.
It needs to be a thick hardened washer that can transfer the load from the bolt stretch into the gear to create the proper amount of clamping/friction.

Imo, it’s one of a few things:
-bolt had been previously over torqued
-bolt had insufficient torque
-substandard washer
-cam was stuck/seized in bearings when you went to refire the motor.

“What I would do” is, remove rocker gear, remove timing cover, see how easily the cam turns.
If questionable, remove it and look for signs of heat in the journals.
If it seems fine, I’d tend to think it was a bolt/washer problem.
If it doesn’t have the OE washer, I’d see if I could find one.
Use a grade 8 bolt, torqued to 50ft/lbs and I put red loc-tite on them.

Leak test all cylinders to check for bent valves.