I would put it under the floor. You can put an access panel with a Dzus fastener in the floor to get at it. If you decide to put it on the firewall, you better have a mockup engine available and installed to make sure you're not putting the brakes where something else needs to go. Under the floor is cleaner too.

My A-bone had it under the floor. My '34 has it on the firewall because it's channelled, and there's no room under the floor. I think you'll find that a suspended brake pedal system is more expensive. Yogi's has a power brake pedal assy for a 48 Chevy for $120, master cyl for $65, and 7" booster for $119.95, two residual pressure valves for $21 each. Total of $347.

Kugel has a firewall mount system for $395, complete with booster and master cylinder. You wouldn't need residual pressure valves with the firewall mount.

http://www.kugelkomponents.com/produ...akesystem.html

Those are just two sources., As you can see, it ain't cheap if you checkbook all the parts.