Grant, that's a very nice setup and it looks dynamite.
The main function of this firewall bracket is to raise the booster five inches.
Why? Because the old OEM booster was made for drum brakes and it needs to be nearly twice as large for discs. The steering column (and A/C plenum) is in the way.
So, Grant fabricated his own firewall bracket using two sections from other brackets, and connected them to a two-stage 8" booster. This booster is the same physical diameter as the old OEM, but it puts out twice as much boost (1,000-psi at 18"Hg with a 1" bore M/C). It turned out nice and he saved money.
Notice that Grant's M/C has crossed Cunifer lines. Each S-10 caliper has a piston that is 2-1/2" in diameter. So, if your M/C has a large and small reservoir, the large side gets plumbed to the front disc brakes. - Dave