Poor idle and off-idle performance after distributor swap

Yes, same issue with old distributor. Good point.

Wild theory here, but there is a ton of play in the distributor drive slot. The shaft can turn by what I would estimate to be at least a couple of degrees. I check the drive tip of both distributors, and they were within spec. I wonder if the engine builder either recycled the old distributor drive gear, or put in a new one with excessive tolerance in the slot.

I always assumed that once the engine started spinning, the drive gear would 'load up' under torque and stay tight..

How much play/slop should the rotor and shaft have when installed in a proper, in-spec drive gear?