master cylinder casting number help

George, I have never performed the SBP to LBP swap but if I were to, I'd probably go with the 73 MC. Whether the larger bore is a factor I can't say but it makes sense to keep the brake system components consistent as with the appropriate proportioning valve. Here's a couple of of MC application tables. May help a little.:dontknow:

Mopar MC Table 1 copy.jpg

Mopar MC Table 2 copy.jpg