That is the most hideous remote brake booster I have ever seen, in fact it looks like two units back to back. This is a principle of braking that goes way back, I remember these types of brake boosters from old Austin buses and other old pommy trucks.
Basically, when you press the brake pedal down it moves some fluid to this contraption in your boot area as a pilot signal, all the action then happens, the booster diaphraghm is (should be) connected to engine vacuum, this controls some valves and pistons etc and by black magic HIGH pressure brake fluid is pushed to your wheels. Here is a similar unit,
Here is australia the VH44 booster has been the popular equivalent for circa 600 years. Pictured below, note the actuating master cylinder on the right.
You could rip it all out and put a firewall mounted booster/ master cylinder combo in. You would obviously require some space in the engine bay, re-plumb some if not all brake hardlines and possibly change pedal setup or ratio. I use a PBR Ford Falcon dual diaphragm booster and a master cylinder from 1970- 1976 model falcon. It's pretty common stuff in other words, maybe your mustangs would provide a good unit to retrofit?