Power brake disc/drum conversion

It matters, you can't interchange the "in" and "out" ports of the proportioning valve. I suspect it would not flow thru if you did, guessing at how the internals work. You want to plumb the proportioning valve to the MC port "for rear brakes". That is the fwd port on all MC's I have seen. Look for photos of MC/boosters in brake kits from Wilwood, SSBC, and others. Most have a "combo valve" right under the MC and you can see how it is plumbed, though most also plumb the front circuit to it as in 1970's Mopars.

To be exact, most newer cars have an "X-split" pattern (fed mandate? my 1996 Voyager does), meaning a LF-RR circuit and a RF-LR circuit. Even if you use such an MC, use the fwd port for your rear circuit.