Joe - I'm a Squarebirder but thought I would offer this idea.
I guess you are trying to add power brakes to a "Little Bird" that did not originally have power brakes?
Use these on the old British cars I work on because many of those cars did not originally have power brakes. Maybe it would fit in front of the battery like the original ones on the "Little Birds"?
We probably have more room in the Squarebird under the dash but have you considered installing a Kelsy Hayes brake booster under the dash? Guess you would have to install the whole brake pedal bracket etc. That's where Ford put the booster on the Squarebird that had air conditioning - there wasn't room next to the evaporator box on the firewall. There was finally a bracket to extend the master cyl and booster out to clear the box but I can see that type bracket wouldn't work on the "Little Birds" with the battery location.
Good luck - I went through converting my Squarebird to front disc's 10 + years ago so know how problematic it can be to retrofit these old cars.........