Modification to a 4 drum master cylinder for use on 4 wheel discsv. ?

Brooks James said

Current set up, factory KelseyHays front discs, drum rear

Plan , rear discs kit from pirate jack, I already have the kit.

I have been unable to find a new factory OE MC for the factory disc/drum set up.

Picture here is a new MC from NAPA
That was listed for 70 Dart disc or drum

I remember a post that to use the 4 drum MC shown here, saw it could be modified to use when adding an aftermarket rear disc kit. I can't remember clearly , something about removing a " restrictor " or a "valve" just before the bosses to install the lines. And also using an adjustable proportioning valve for the rear ?? .

I am trying to keep all my speed and stock parts "era correct". I know about Dr Diffs MC kit with firewall mounting adapter, which, being aluminum and plastic is definitely
Not era correct.

Does anyone remember the post, or know what to do ??

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DON'T use a drum/drum MC with disc brakes. The (rear) reservoir for the front disc brakes will not be large enough for the front calipers. Use a disc/drum master (Rock Auto has them listed- 1972 DODGE DART 5.6L 340cid V8 Master Cylinder | RockAuto ) and remove the residual valve from the (front) reservoir for the rear brakes; just run a sheet metal screw into the brass seat and pop it out- the residual valve is right behind it, then reinstall the seat. The (front) drum reservoir is plenty big for 99% of the rear disc conversion kits out there, rear discs have a relatively small caliper volume since they do comparatively little braking.
The drum/drum part that you're probably thinking of is to use the drum distribution/proportioning valve since the bias remains fairly accurate for a four wheel disc system- more so than the disc/drum unit; although with the Pirate Jack rear disc system you'll probably need to run a separate proportioning valve in your rear brake line anyhow.