[yossarian19]

From experience I can say the CPP Firewall kit fits pretty well, with two caveats:
1. The spacers that account for the ribbing in the firewall are a little hokey / annoying to install. Further, the firewall flexes when you hit the brakes hard enough. The whole mess needs an 11 ga panel, about 8x12 might do it, welded to one side or the other of the firewall to reinforce that area. So "mounting" is a big "meh" on that kit, but it does work. Hell, I didn't even take the seat out or the fenders off and I got it in place & driveable.
2. The brake pedal arm is mounted WAY too high. I modified the booster rod to lower the pedal height into "acceptable" but what should really be done is to grind the welds off, re-clock the pedal arm on the shaft & burn it back home.
Other than that.. no complaints. Each and every OEM puts the MC / booster on the firewall. It's not an accident... it's the right place for it. Obviously an on-frame mount can work just fine but again, in my experience, it's a PITA. I had an under the floor pedal setup and got rid of it. Never fit quite right, clearance issues, maintenance was a bear, etc.