Dating previous owner's disc conversion

Good question. I'll check on that when I get into it this weekend. Switching the spindles would be a good opportunity to change out all the bushings and any bad ball joints. Is there any braking disadvantage to having them on the back side of the wheel as opposed to the front?

The bleeder valve would stay on top, I think.

No braking disadvantage to moving the calipers to the backside, just make sure you use longer brake hoses. The bleeder valve does stay on top if you switch the calipers side for side with the spindles. If you don't swap the calipers side to side as well it does not.

Does anyone know about the compatibility of the 1973 power brake MC?

As in, you want use a 73+ cast iron MC? It would work fine. But the one you have already is better. If the caps leak just change the seals out. The real mess you have is that thing underneath the MC, I don't like that at all.