I think you will find that prems could be optioned up with 4 wheel discs. You must be looking in the wrong places because WB statesman disc brake set-ups aren't that hard to find.
Doesn't matter what ratio he finds as it will all bolt straight onto his existing 3.36 diff.
You can also bolt a Commodore disc brake set-up onto a Salisbury, all you need to do is have the backing plate machined down where it bolts behind the axle to the same thickness as a drum brake backing plate so the drum brake bearing & seal works. Everything else will bolt straight up, all you need to do is get a custom handbrake cable made up. I did this many years ago on the back of my Torana with a Salisbury, worked a treat (Now i run Commodore axles, so no mods are needed).