Mocking Up Firewall for Master Cylinder Studs

I would just prime it with metal etching primer and cover the oops w/ fiberglass, or you could use Bondo Hair (has chopped fiberglass) then regular Bondo. There is no structural concern.

I think using your cut-out part as a template is fine. 67Dart273 spent a bunch of time making a template for you, so verify against his. I don't think it matters much if your brake centerline is off 1/8" as long as the bracket doesn't bump into anything under the dash and your pedal doesn't wind up in a funny place. Since you will use a 2-bolt MC, you probably could have used your original bracket w/ missing studs and drilled 2 new holes for your MC. That is what I plan for my 64 Valiant. If that doesn't work for some reason, I will just use a 4-2 bolt adapter since I still have my 4 studs. If you bugger up your 2nd brake bracket, I have one w/ pedal from a 74 Dart auto I'll never use and I will be passing near Covina next weekend. PM if you need it. Real cheap.