If it's under the floor it's not on the firewall. A 39 is supposed to have the MC under the floor right??

I would make a bracket like ABS power brakes sells (link above), it's very simple.
I modified my pedal bracket to fit the hydrobooster, would have been simpler to use the separate bracket mounted on the frame.
http://rides.webshots.com/photo/1484168811080251109bfKirL

Even if you don't buy anything from Hydratech do check out the tech section and the FAQ, you can learn all you need to to then pull your own parts from the junk yard.

Brakes are not a place to cut corners, don't be to cheap and end up with an unsafe and unreliable system.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-