Most modern master cylinders use two mounting bolts. Your original booster and firewall holes use four mounting bolts. Unfortunately this modification cannot be done with the under dash booster. I finally had my booster modified to accept a 2 bolt style master cyl. Hopefully Dave will see this post and chime in with better info. You would still have to split the brake system but not sure if you would need the proportioning valve. I believe the easiest route would be to install one of the newer dual action boosters and brackets under the hood with the dual drum master cyl already attached and matched to the booster. I would say that if the master cyl you choose comes from a system that has a proportioning valve you will need to add one to the Tbird. That dual circuit system might now require a proportioning valve to balance correctly. When doing this you would most likely split the current common one line system into a separate front and rear system. Also your old master cyl to booster rod may or may not be the correct style and length for the new master cyl. Not sure that will work when leaving the old booster under the dash but all I can think of at the moment. Might be able to remove 2 of the old studs and install new larger ones to mount the new style master cyl. The problem I ran into is that every master cyl I looked at (like the one you show) had 2 bolt mounting tabs and the Tbird has a 4 bolt mount. Mine has the external booster in the engine compartment but the 4 bolt pattern on the end of the booster in the engine compartment is the same bolt pattern on the firewall with your booster under the dash.

I tried to do the same thing with mine years ago. Mounting the master cyl and the master cyl rod lengths will be the biggest obstacle. For instance - my friend has a 1970 Ford F-100 with 4 wheel drum brakes so that might be another cross reference option. In theory - you should be able to pick a vehicle of that era with 4 wheel drum brakes and the master cyl would be compatible with the 4 wheel Tbird drum brakes.

Haven't used that particular one but you may be able to make it work.

59 ford t bird