From all I've read, you only need to change the mustache bar and lower transverse crossmember to use the later style diff mount. This seems somewhat consistent with the microfiche data, which shows that the diff front crossmember, mustache bar, and diff mount changes in 7/71, but the driveshaft doesn't change until 9/71 (which is coincident with the change to the "B"-series transmission). Not sure what the change is from diff front crossmember 55411-E4100 to 55411-E8300, but maybe it didn't change in ways that matter as far as the diff mount goes. MSA also seems to agree with this change set, FWIW.
In any case, I will eventually be testing this as my 5/71 rear suspension is currently set up with the original diff front crossmember and driveshaft, but later diff mount, later mustache bar, and later lower transverse crossmember. "Original" with respect to the diff front crossmember and driveshaft is assumed, as anything could have been changed in the 20 years before I took ownership of the car, but the other rear suspension pieces that came off the car are all obviously series 1. The rear suspension is still sitting on a rolling cart, so maybe I'll find incompatibilities when I eventually install it back into the chassis. I have all the original pieces, so I can revert if necessary, and the original series 1 diff mount looks pretty good. I just figured I'd replace anything with 50-year-old rubber. I'm sure there are others in the group with more knowledge in this area, so I'll stand corrected if any of my assertions are incorrect.