brake proportioning valves questions

A drum/drum car has no proportioning valve. Which makes me think you are talking about the junction box/safety switch. Which, in a 1974 disk brake car, includes the proportioning valve. But in a 1968 car, the PV was plumbed in separately on the rear line, so the junction box/safety switch would not have a PV either way.

In any case, the proportioning valve isn't really affected by the master cylinder size.