proportioning/metering/distribution valve clarification
Is this what you got?
PV6070MOP - O.E.M. Mopar Proportioning Valve
which looks the same as this but viewed from the other side

I agree it looks like it's for a B-body (or e) after Jan 1970
1970 Chrysler Imperial Hydraulic Brake Service Guide from the Master Technicians Service Conference Session 274
So then the two questions are:
1. If the metering valve is seperated, is there room to mount the combo switch/valve
and is the repop version made with a plug that can be removed? (see page 12 of the 1970 MTSC Brake pamphlet ) If not, then it would be an experiment to use the metering valve and see if the delay to the front is acceptable.
2. Is the proportioning the same for the 1970 floating disk cars as the earlier versions?
According to pages 10-11 seems like the answer is yes.
If this is all too much a PIA, and especially if you're not going to use the brake warning light then you can just plumb in a seperate proportioning valve and be done with it. This is assuming that your tandem master cylinder curently feeds a distribution block with one line out to the rear wheel cylinders.
You could then just use the 'Texas' version, or the Mustang style cylindrical version or an adjustable.
Yes, you'll have to play with the adjustment a little but that's not hard. You can get the adjustable ones in different sizes and shapes. Direct Connection/Mopar Perfomance used to sell a cast one made by Kelsey Hayes. It was pretty big. Then they sold a small one I'm pretty sure was made by Wilwood. Maybe the smallest out there now is made or branded by Russell. Lots of choices in between. They all work about the same. You'll still have to make a bracket and/or install a thread-sert into the frame rail or other spot that's convenient.
If you make your own connecting tubes, using the coil armor will add some protection from rocks etc (just like the factory did). But getting OT.
PS. For fixing the quotes. Go into edit and look for a missing bracket or something in the [/quote] which should end a quoted section. Hope that makes sense