Although I built a Classic Roadster, that was quite a few years ago. And I don't remember how the pedal box is put together.
You will need to shorten the pedal, but that's at the top. You need to move the pivot point down about 2" or so. If there's a hole there in the box, then that's how much it should be shortened. The attachement point for the brake rod, and the pad stay in the same place.
That makes the total pedal length 11.75, and the pivot to rod length 1.75.
11.75 / 1.75 = 6.7
Still not perfect, but a dramatic improvement.
My car has a pedal ratio of 6, and 3/4" MC's.
I started out with stock Mustang and T-Bird components, and I had no trouble locking the brakes at any speed less than about 40. Now I have Wilwoods for track use, and braking is even better. But the stock components were more than adequate for anything less than high speed track abuse.
If you're going to modify your pedal (and it really sounds like you need to), then you should also consider making it a dog leg. Put a 7* angle just below the pivot point. I did that to my pedal during the initial build, so I can't say it made any improvement. I "borrowed" this idea from Richard Oben.
http://www.norcal-cobras.com/project...rake_pedal.htm