HELP with master cylinder 4 wheel disc

Jhenry70,
Good plan and good research. The block in your Duster is a "distribution & imbalance warning" block. It does not have the proportioning (rear) and metering (front) functions of the "combination valve" on factory disk cars. Yes, you can re-use it, and should since it keeps your dash imbalance warning lamp working. Yes, you also need an adjustable prop valve to the rears ($30 cheap), and it must be after the imbalance switch to keep that working correctly (i.e. install it down in the mud). You could also replace both with a combination valve, but pricey and non-adjustable.

Your drum MC would actually work. It just doesn't have as large a reservoir for the disks. Also, some have a "residual valve" on the outlet ports that would need to be pulled out so the disks don't drag. Many MC's would work. I am putting a new one for a 99 Breeze w/ ABS ($25) on my 65 Valiant w/ manual brakes, via a 2-4 adapter plate ($30). Many use an 80's Dodge truck MC w/ same adapter, which is what the Dr Diff setup looks like. You could buy the same parts cheaper on ebay, but his price isn't painful. I abhore rust in my brake components, so use silicone fluid in all my non-ABS cars.