I did this conversion on my 65 Barracuda. My understanding is that you don't need a rear residual pressure valve if your rear wheel cylinders have the expander cups in them for this function (those cups have been common in rebuild kits since the 70s) I put a disc/drum mc on the car before I did the swap and it worked fine.
I did use a rear proportioning valve b/c a super hard panic stop caused the car to swap ends .
I don't know enough about metalurgy to know what type of steel the adapters are made out of, but I can tell you that they are STURDY. If anything, they seem to be over-engineered.
