Found parts that are pretty much bolt on. You have to knock out the "wedge" that's fitted to the center pin (that the top of the shoes ride on) on top of the backing plate. Then Jeep Gladiator dually rear shoes fit right up. The wheels cylinders for the same vehicle also bolt right up and use a 7/16" threaded fitting. You will need the hardware kit for the Gladiator as well as the upper springs attach differently.

Luckily, these parts are all the same as Chevy duallys of similar vintage to they are pretty easy to come by.

Raybestos numbers for the cylinders are:

Left-WC9344
Right-WC9345

Raybestos Shoes

60PG

Raybestos hardware

H7027

All on Rock Auto for less than $100 total.

I test fitted my original dually shoe to an M715 backing plate and it fits fine once the "Wedge" is removed. Hold down holes line up and the shoe rides right on the flats stamped into the backing plate. I test fitted my old and new wheel cylinders up and they fit perfectly. Mounting holes line up perfectly.

You could probably get away with using the wheel cylinders with the stock M715 shoes.

FYI, Gladiator duallys used 13x2.5" rear brakes, just like the M7xx series trucks and have no self adjusters or provision for mounting a parking brake.