Axle conversion?
You'll want to reference the brakes that you have by the drum size, not the backing plate size. 11" drums use backing plates that are 12 1/8" in diameter, 10" brakes use backing plates that are ~11". Since usually the rear drums are referred to by the drum/shoe size, using the backing plate size gets confusing.
What you need is any 10x2.5" drum brake set up, as the 10x2.5 rear drums only came in BBP. Or an 11x2, 11x2.5, or 11x3" set of 11" drums, all came only with BBP. But with those you have to make sure you get the right shoes and drums, because the backing plates have different offsets for the different width's. Easier to buy a whole set rather than just backing plates, because then you have to figure out what fits, ie, you can't put 11x3" drums and shoes on 11x2.5" backing plates.
Check out this thread too, it shows how to measure the offset to find out if you have BBP or SBP backing plates...
http://www.forabodiesonly.com/mopar/showthread.php?t=258919
As far as these,
I wouldn't touch them. Those are sbp axles that have been drilled to BBP, and you have to use sbp drums that are redrilled as well. Since the SBP drums are getting hard to find, that's not a solution, that's a headache.
If you want to just buy a set of backing plates, take a look at this set of 11x2.5" brakes. I've purchased from this seller before, they have pretty good prices (not much more than the wrecking yard), ship quick and are great to deal with.
[ame="http://www.ebay.com/itm/11-2-5-PLATES-LOADED-MOPAR-DODGE-PLYMOUTH-8-3-4-DRUM-BRAKES-8-75-NEW-YORKER-/281211364925?pt=Motors_Car_Truck_Parts_Accessories&hash=item417980e63d"]http://www.ebay.com/itm/11-2-5-PLATES-LOADED-MOPAR-DODGE-PLYMOUTH-8-3-4-DRUM-BRAKES-8-75-NEW-YORKER-/281211364925?pt=Motors_Car_Truck_Parts_Accessories&hash=item417980e63d[/ame]