Don't believe MasterPower is currently offering the 11" rear drum "upgrade" kit, has been asked about before. If you're dead set on going down the 11" rear drum path, not that hard to source a removed pair of rear backing plates from a '75-77 GP rear or other late '70's early 80's 11" GM B series rear drum car. Both parking brake cables exit forward at the bottom on these, unlike the passenger side on the later '80's B-body version which has a rear exit on the pass side backing plate. Rebuild with new brakes shoes & small parts. Cheap 11" brake drums are avail new for around $40-45 a piece. In the past, putting such a swap together for a customer, I reused original 11" smoothie style drums off a late '60's Impalla for this swap.
Personally, on my own Pontiac restorations, am not a fan of the 11" drum rear swap. I also like to able to swap on 9.5" diam alum rear brake drums. Less weight/less mass. Thus years ago, when the company was still known as PraiseDyno Brakes I began to order their brake in box true upgraded 9 1/2" diam drum brake kits. For the last 15 or so years, the company iscknown as
www.musclecarbrakes.com. These kits include special high matrix compound brake shoes, stiffer springs, along w 1/16" larger diam wheel cylinders. Not a cheap entry level drum brake rebuild kit, but the combination of parts provides quite a bit better braking /lack of brake fade. In swapping over to the musclecarbrakes kit, there are 3 loose stamped steel brake pieces that will need to be reused from an original drum brake asm. Also the four T shaped "rods" that that go into the wheel cylinders. If yours are badly pitted, can buy those small pieces new from Inline Tube. There is also one new stamped steel trailing bracket & small hook bracket that will test most anyone's abilities to assemble the spring on. Took a while to solve the latter issue.