[bobcowan]

The most common cause of spitting belts is a slight misalignment of the pulley. I would start by checking that.

If it's happening under "violent" acceleration, I would guess that the torque is causing the pully (and alternator) to become misaligned. In that case, you need to support the alternator and bracket well. A good installation would include a bracket from the back of the alternator to the cylinder head or block. If all you have is a C-arm on a pivot, that's not going to work.

Look at a good bracket for an early 70's Mustang with a 351W.