krazy texan - for your 68 master cyl:
- the FORWARD reservoir (toward your grille) should be plumbed to the REAR brakes.
- the AFT reservoir (toward firewall) should be plumbed to your FRONT brakes.
Counter-intuitive, I know, but the dual drum master cyl is designed such that it biases front, but only if you hook er up correctly. No proportioning valve is required, but you can add one if you have a strong desire to fine-tune the brake bias.
Hope that helps. Chief Rocka hooked me up with the definitive answer to the same question I had a while back.
EDIT: here's a link to the referenced thread where Chief provided a great diagram of which mc port goes to front/rear brakes, etc.
http://67-72chevytrucks.com/vboard/s...=332148&page=4 see post #80 specifically (Thanks Chief!)