Starting from top>down;
Master Cyl:
If using a proper disc/drum dual circuit master, you should notice that one of the reservoirs is larger, this is the one you use for the FRONT DISC circuit, the other, smaller reservoir is for the REAR DRUM circuit.
Brass Block or 'Diverter' used just for the FRONT;
If you are referring to the basic 4 wheel drum block and it is no more complicated than a brass block with 4 machined openings for inverted flare fittings, the following should apply:
The standard block will not have any proportioning, all openings are common to each other and non-metered, so it makes no difference where you plumb the in/outs but it might be best to duplicate the original set up to simplify the front disc connections.
On my '66 BBody uprgrade from single pot master and 4whl drums what I did was install the block in the standard location so the front lines (3/16") still fit up like original and then simply capped off the old outlet for the rear with a 3/16" flare plug. This made the block a left/right splitter for just the front with the inlet from the master coming in the top opening (which is larger 1/4" I think). This made the front a separate circuit using the original drum diverter block and OEM front line locations and sizes - one outlet came out the rear (towards firewall) one came out the front (towards radiator).
For the rear with new Proportioning Valve:
Now you just need to determine the best place to mount your new prop valve for the rear. I think it best to put it down past the trans linkage stuff somewhere along the lt front frame rail. To connect that, just run a new line down from the smaller reservoir on the master making sure you stay clear of trans linkage and exhaust, etc. and connect that new line to the 'inlet' on your prop valve. From the prop valve 'outlet' you run a new line back to the rear or if your existing is minty, you can trim it to fit with a new double flare and you're all set to bleed and test. It would be wise to have a 360* loop or two in your new lines coming down from the master to take up flex/vibration and don't get too frantic about the position or orientation of those loops, if they are below the level in the master and you bleed properly they will not trap air bubbles or black helicopters or other bad ju-ju.
