Chris,
I really don't like that idea if there is any other way possible to have mechanical linkage to the booster and MC.
A major advnatage to upgrading to a dual circuit MC is safety, if one line blows you still have reasonable chance of stoping.
If you have the original MC operating the new one with a
single line you have gained nothing for safety and added the complication of another hydraulic system to tend to.
That said I have done (in an old ford) exactly what you are considering. At the time I couldn't come up with any other options and afterward every time I drove the truck it bothered me, very mindful I could loose brakes at any moment.
https://picasaweb.google.com/lh/photo/HS0Ler0FXfwQyvaIaOoQEdMTjNZETYmyPJy0liipFm0?feat=directlinkInstead of that approach I don't know why you can't use linkage from original pedal back to the booster mounted in the frame?
Or if no room can even be done outside the frame. Pictures in this thread nicely illustrate some possibilities.
http://67-72chevytrucks.com/vboard/showthread.php?t=643079