Single to dual M/C. Some specific queries

I re-used the distribution block in my 1964 and 1965 Mopars (Valiant, Dart, Newport). The existing feed from the MC is a 1/4" tube to the top port of the block. I put an inverted flare adapter (1/4 M to 3/16 F) in that port since all dual MC's use 3/16" tubes. I plugged the port to the rear brakes. I used an inline coupler to connect the new 3/16" tube from the front MC port to the existing rear tube. I plumbed an adjustable proportioning valve ($30) to the rears in case I later switch to front disks. All adapters were bought in bubble packs at Autozone. I posted the PN's in the past, if you care to google.

In all 3 cars, I used an MC for a 1999 Breeze w/ ABS. One MC is mounted to a 1990's booster for an Intrepid, mounted on Dart firewall brackets (avatar). Another has a 2-4 bolt adapter plate to mount to the C-body booster. Another uses the adapter plate straight to the firewall (manual brakes), using the existing brake rod (w/ new rubber lock bushing) and the pedal wound up in the same place. Most people here use the 1980's Dodge truck MC, which Dr Diff sells w/ adapter plate and custom smaller bore (for easier pedal). The Breeze MC has a 7/8"D piston and 2 regular inverted-flare ports. I wouldn't use the 1967 MC since they slosh fluid out to remove paint from the firewall, and thus let moisture in, plus cast-iron is heavy and looks ugly to me, but some people prefer them as they do their wives (avoid those photos).

I don't have a pressure-imbalance switch in any of mine. I'll bet most vintage ones had the piston rust in-place to no longer work (my 1969 Dart gave no warning). You can buy new "combination valves" with one (~$70). If you do, wire it in parallel with your e-brake lamp, to connect to ground when actuated, which you can also do for a reservoir level-sensor. That is how my 1996 Plymouth is wired. Any of the 3 will light the "brake warning" lamp.