Manual disc brakes, master cyl?

You didn't say you were using 4-wheel discs. If not, you probably do want a proportioning valve, because drum brakes increase braking force disproportionately with pedal pressure, while discs are proportional -- this means in a panic stop the drums will tend to lock first. What 318willrun said is correct, in that the factory equipped the small (15/16) bore with power disc brakes, and the big bore (1+, depending on year) with manual disc brakes. Summit makes a reasonably priced proportioning valve, as well. Tip: to install the PV without having to cut and flare lines, buy a short section of generic brake line with the correct fittings at your local auto store (16" or so will work) and bend it into a loop. Then mount the PV directly next to the safety switch block. Just undo the rear brake line output from the block and into the output of the PV. Then install the new loop so it connects the input of the PV to the now vacant port on the block. I see a lot of people installing the PV between the master cylinder and the safety block, but the factory put it after the block in the line going to the rear brakes. I am guessing this was to avoid the operation of the valve affecting the brake warning light, although that probably isn't critical.