Caliper Bracket

You shop is likely thinking of the difference of bearings for each side, or even the axles. That shouldn't be an issue functionally, just in servicing those parts. Any non stock parts can be a problem for new owners, or after many years, and people forget.

It would be very wise to create a record of all changes from stock parts, for every vehicle.

For the brakes, I still am very interested in learning how hard it will be to swap a common late model rear disc system to the old 70's Fords.

The 03-2011 Crown Vic, Grand Marquis, and Town Car, all have the same rear brakes, 12" vented discs. I think the backing plate can be easily fitted to many other Fords. But the axle may need to be custom ordered, for proper length etc. Plus the parking brake cable goes into the backing plate at 90*, with a mounting bracket on the diff housing. I see a need to weld that bracket to any other car's diff. I have those brakes(the parts) from years ago for my old mail truck, and didn't make time to try it. I will soon, for my Mark VII, or my Ranchero, whichever I get to first.