A. Mopar cam bores have a tendency to run small. It is not a new problem. It can be solved with a hone.

B. Core shift, no, because machining is done after the casting process. BUT, the blocks change shape as they age and the internal stresses are relieved. It is not unusual for a cast iron block to have a couple of thousandths of movement from stress relief. Casting shift would be more like it.

C. The textbook way to take care of a tight bearing is to blue it, test fit it, then scrape the parts of the bearing that rub. It can be done very artfully, however it's much more difficult on a cam bearing because of the lack of space in which to work.

D. The slotted cam journal works, too, but may leave a rougher surface, although it doesn't seem to make a difference to the engine.

E. Machining down the diameter of the cam journal to get a proper fit should be pretty easy for a decent shop, as another way to fix this.

F. I cannot imagine a machine shop so helpless as yours sounds like. Maybe a trip to OKC is in order, or else to a better local shop? They do not sound like people I'd trust with my numbers block.

Good Luck,
R.