Standard manifold?
Would a different carb setup produce enough room? Not sure what you have.
Is there a particular feature that interferes on your chosen master cylinder that may be elsewhere on a different model fitment? If it's the reservoir, can you rig a remote reservoir?
Other admittedly wacky options if dropping by an inch doesn't do it:
You might be able to cheat the engine forward a small amount, by using re-drilled motor mounts, offset plates, or the like. Transmission mount may accommodate spacers at the transmission-to-plate location. But I don't know if 1/2" or 3" would be required to clear things if moving forward. If it turns out to be 3", for sure you won't achieve that unless you ditch the fan and do other extreme surgery. The driveshaft will allow a little movement, but not 3", although a spacer at the diff may let your preserve some travel.
Another place to look is the master cylinder. I've seen some with 1 1/2" thick mounting ears. 3/4" would probably do fine, so if the ears are thick you could mill away some thickness and have it hug the firewall better.
If the body of the master cylinder is pretty long, you could mount the ears inside the car instead of outside. Cut the firewall hole in a vertical "ovaled-diamond" to clear master cylinder ears, and install the master by inserting it clocked 90 degrees, and then turning upright once inside. The bolts won't care. Lots of other firewall items will have to be removed temporarily, and new pushrod fabricated. Do some measurements/calculations on pushrod length, throw, and what inside mods would be needed to accommodate this. I would think there's a 5% chance there would be enough room in the vicinity of the pedal hangers to pull this off. I haven't been under the dash in 3 years, so confidence is low.
The thing to remember is these last two are decidedly inelegant, and tear up your car. Definitely not "leave no trace" mods.