If you are not concerned with originality, there is an old master cylinder hack for these cars.

IIRC, the dual pot M/C from a '68 Dart (manual brakes) can be used in place of the original, and plumbed in with couplers to front and rear lines. If done correctly, this will give the added safety of separating front from rear brakes in case either should fail. Double check piston size, etc. but should be enough for the 12 inch brakes the 300 has, if they are original.

You should also be able to incorporate factory power brakes as well, as donor booster set ups can be found on most any full size 62-64 Mopar full size. You can still use the '68 M/C with these.