Is this true for 300's as well?
No. Imperial didn't it's own thing for a long time.
As for the 75 Imperial rear disc brakes, I'd run and hide before I put them on a car. You can forget about ever getting replacement or repair parts.
Too many people just don't know much about those rear disc brakes, in this day and age.
Yes, the rotors would be difficult to obtain, but if the are in good condition, then there is no need to replace them.
The calipers that are used, are nothing more than 73-76 A body calipers, which there are a bazillion of them out there.
And the pads are the same, and all that you have to do is take a grinder to the portion of the metal "lip" at the edge, and grind it away.
Easy, peasy.
And if the lining on the parking brake shoes ever wore down, just get them relined at a shop that is capable of doing something like that.
Then everything else in those brake systems never need replacement.
