The 11x2.5" drums fins & all will clear standard 14x5.5" Rallye's with room to spare. There is room for more backspacing if You have custom width items, but it would be
really tight to the fins, I can't say whether it would affect heat dissipation negatively or not.
Couple of questions;
1) What width shoes did You install, 1.75", or 2.5"?
2) Is this drum to backing plate gap identical on both sides right now?
3) You stated "The original axles were BigBoltPattern, not sure about the donor car", and "this was an original BigBoltPattern setup, but one of the backing plates was
damaged". Where are the drums that were on it to begin with? or did You purchase the 8.75 sans drums/brakes/hardware?
My observation, You probably just need A) the right shoes if they're not the 2.5" ones(they appear seated, square, and the w/cyl. struts look straight?),& B) 2.5" drums.