Heres a sneak peak, if this gives you any ideas for mounting the pedal box.
Due to the curve of the firewall, if you raise the pedal assy up about 50mm you can go forward about 50mm without cutting the firewall, but by raising it up that much means you will clear the lower part of the firewall double skin section, and once you cut a hole, theres a nice cavity from factory just waiting for your master cylinders to go in.
Looking at the pic, the greenish primer panels in the cutout on the back, right side and bottom are there factory, i have not added those, the left side which you cant see in this pic is just a void, no side wall thereuntill i guess closer to the fuel tank tunnel.
Also the AP Racing CP2623 extra short master cylinders are the way to go, they are very short and have the outlet on the top, not at the end like most, which that alone will make it shorter.