How to identify a master cylinder

My dilemma: I just upgraded the car from 10.7 slider front disc to 11.75 pin calipers. The sliders are 2.6" pistons, and the pin calipers are a larger 2.75".

I can already tell the pedal feels spongy, and I knew the master cylinder might require replacement, but how do know what I have?

My car was originally a power brake car, but somewhere along the line it was converted to manual brakes, but I don't know what master cylinder was used in the conversion.

I haven't taken it off the firewall yet, but as yet I don't see a casting number. Where is it typically located?