This kind of problem isn't uncommon. I have had to install two different brands of canti arms on repairs before because the bosses had different C-C dimensions, front compared to rears. One Bridgestone had rear posts about 82ish mms apart and the fronts were closer to 70ish. I tried about 5 different cantis on the front before I found the least problematic and that needed no further mods. This also was motivated by a rim width increase (new pair of rims) to support wider tires.

At some point the efforts and mods to fit more current parts on bikes designed around older understandings become more than the rider wants to do, or pay for. Andy.