Richard, I haven't seen anyone use a proportioning valve to the front, but in theory it seems plausible.

69, before trying some creative idea I'd go through the existing system first and bring it up to snuff. You originally mentioned that the car sat for some time. My guess would be that you're having problems as a result of corrosion. Brake fluid is hygroscopic (attracts and retains water), so the wheel cylinders could very easily be hanging up not allowing any modulation. Also the shoe/pivots/springs/drums could all be corroded and causing bind. Doing a system flush goes without saying (although I just did didn't I?) As for parts, I gave you my guess earlier.