Use porportioning valve with drum/drum brakes

Recently ordered MCPVS-2 for 63 Bel Air currently with drum/drum brakes (newly rebuilt with Kelvar shoes). Would like to use MC/PV with these brakes for time being. It will be a while before I can change out to front disks and I would like to plumb this setup only once. Also, would like to be able to use brake pressure switch. Can PV be modified to allow front end to serve only as a distribution block for front brakes? Have also read that a PV can be used with drum/drum when there is big rubber out back as they provide control and stability to keep rear end under control (245/45 on 8" rims). Also, on the front of my PV there is a small pin under a removable rubber cover that can be depressed. What is this for? Regards and thanks in advance for your thoughts and suggestions, Don