Scott,
Rather than installing another proportioning valve in the front line, couldn't you simply connect the one you have now, backwards? That would put the proportioning valve in the front line and leave the rear wide open. Even though your balance would still be way off, at least you could get the fronts to stop locking-up.
As a better alternative, don't wait for Baer to have their rear "kit" available. Just buy a pair of calipers from them that match your front calipers, with the piston sizes they recommend. Then have the appropriate adapter brackets made. You already have upgraded rotors on the rear. I'm sure SACC could help you out with custom adapter brackets. Just have the rear calipers shipped there (to SACC) first. If you're determined to wait for Baer, just buy an interim set of rear calipers. Maybe SACC would have something used that you could sell on eBay later on.
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.