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Proportioning valves -- or in the case of the stock one, a combination metering/proportioning valve -- are the LAST thing you want to change arbitrarily.
Been through it a few months ago... You don't need a proportioning valve, I took mine back after buying it. Here is my old thread. Read the final post! And the others if you need more info.
My dad just put disks on his 14FF, 1974 K20, setup works fine with no mods to anything else, didn't even use the factory rear proportioning valve. (I talked him out of that )