Proportioning valve / rear brake problems

Since the R-valve is already plumbed, (correctly I assume;they are directional), I would leave it where it is. There's no telling what might,or what might not,currently be in the old rear wheel cylinders. Doubling up is ok.
Yes it's very possible to not need a P-valve with 9"brakes out back, and discs up front. It's also possible to not need one with 10"rear brakes and 295s. Lot's of things are possible. But the OP already has a P-valve installed. It may just be a matter of swapping two lines to get it functioning properly.
Why make him redo all his plumbing to satisfy a possibility. And his P-valve appears to actually be a C-valve, which turns the dash lite on in the event of a pressure differential across it. Here in MB the dash lite has to be functioning properly,so he would be required to plumb a safety valve in there instead. Why make more trouble for him than he already has? And since I am licensed to perform safeties here in MB, the brake system is one area I scrutinize.