Dr. Diff 10 x 2 1/2 inch or 11 x 2 1/2 rear drum brake Assy

I would say,depends on wheel/tire size.
15" wheels under 10" width.
Tires less than 28" diameter 12" cross section.
10x2.5 should be fine for stopping.
A larger combination, you may need 11x2.5.

Having run the 11.75"/11x2.5" combination with 225/60/15's, 245/60/15's, 275/40/17's, 275/35/18's and 295/35/18's, I wouldn't keep the 10.95's and 10x2.5's with anything 245 or wider. Will they work? Sure. But with a good tire compound the 275's can push the 11x2.5's to their limits even on the street. Same for the 11.75" front disks with 275's. When I was running 11.75" front disks and 11x2.5" drums with semi-metallic brake compounds on my '72 Challenger you could tell they were working hard to reign in those 17x9's with 275/40/17's, and those tires were much harder than the 200 tread wear stuff I run now.

And again, the 11.75 front disk/11x2.5" rear drum works with 15" rims just fine. Probably pretty close to as big as you can go for brakes with 15" wheels. And they're not crazy with 225/60/15's either, they're still an improvement over 10.95's/10x2.5's.