Firewall & master cylinder don't match?

The car: 68 val
Original brakes: 9 in drums on all 4 corners
Upgrading to: front discs and larger rear drums

I never paid attention to the way the old (drum only) m/C fit the firewall, but with the new disc brake m/C the flange that bolts to the firewall just BARELY covers the cutout in the firewall for the brake rod.

Anyone ever encounter this? Did you do anything about it?

I am concerned about possible water leaks. I thought about making a sheet metal "washer" to go behind the M/C and smoosh a larger homemade gasket against the firewall. But thought I'd ask if anyone else encountered this and solved it first.