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

I'll say for power disk, the later booster may be easier and cheaper to source.
The correct master cylinder probably is also slightly more available, and certainly alum/plastic ones with the correct bore for the later system are.
However, the later booster may have been designed with a different pedal ratio in mind. Not sure '73-74 but later ones pretty sure did.
All stuff people have dealt with before so its hardly uncharted teritory