Since you are using the under floor master and booster, do you have any residual valves in the system?
Seems in the back of my feeble mind I had to install a 2lb residual valve for the front and a 10lb for the rear (disc/drum) when I was having pedal problems on a 50 Chevy I built.
The residual valves helped a lot.
But maybe others with better ideas will chime in soon.
