Here's an earlier thread you may find interesting:
http://67-72chevytrucks.com/vboard/s...=dash+cylinder
And here's a link to one manufacturer that produces under dash systems:
http://www.kugelkomponents.com/produ..._reverse90.php
Personally, the only real disappointment I have with my 1960 GMC is I installed the Ron Francis fuse panel in the conventional location to the left of the clutch pedal. I'm old now with back and knee problems, and it's an exercise in torture if I want to work in that area. I wish I had put it behind the glovebox with a trap door so I could work on it comfortably sitting in the passenger seat.
Regarding the under dash master cylinder -unless there is some means to remove the top of the dashboard for access, I think it's going to be a miserable experience trying to bleed the brakes, or check the fluid level. That and the risk of ruining the carpet during brake maintenance or should a leak develop. If I wanted a clean firewall, I'd be more inclined to go with an under floor design like commonly used on earlier vehicles.