Decision: 5x4 (SBP) or 5x4.5?

As long as there are caliper parts for mustangs and corvettes, there will be parts for mopar K-H four piston calipers.
The availability of the mopar disks was very limited for a while, at least decent ones. Definately pick up a spare set when you have a chance.
Same with the drums. It was the rear drums that went unavailable new for a while - specificly the 10 x 1.75" drums with SBP.
10 x 1.75 drums with BBP had even less original use. Those are probably harder to find at times. Kindof like the Bendix 4 piston brakes used just a few years on b-bodies.
Point is, on older vehicles, parts available is never guarenteed.