That seems to be a common problem. I was just fooling with my 65's booster last night. I got a new one from rockauto, but it looks like they clocked the 2 sides 90 deg off, so the bolts don't fit the firewall plate. Unfortunately, it is the Bendix(?) booster which crimps the 2 halfs together, instead of the Midland-Ross (my original) which has an easy to remove clamp collar.
Update: my mistake, the rockauto one does fit. I was comparing it to my original booster and I am the dumF who clocked that baseplate wrong. Just fixed it.
Last Fall, I installed a booster from a 74 Dart, because I had it and couldn't find the right booster affordably. Though it looked almost identical, the bolt pattern differed. I tried swapping the back-plates, but found the piston diameter thru the back plates differed. I ended up drilling new holes in my back-plate. Currently trying to go back and switch to a dual piston MC for safety, plus an aluminum 2-bolt type (requires $35 adapter plate).