Larger brakes serve several purposes and add a few things...
The 11" diameter means the braking force is greater even if the shoes are the same size. This is because there is a greater moment arm acting on the axle. It may only be 1", but it means that your braking force gets multiplied by 5.5 instead of 5, which is a big difference. I haven't put the shoes side by side either, you may get more area as well, which also increases the braking force. For example, lets suppose the shoe area doesn't change and you have 100 pounds of braking force a the drum. This would give 500 inch/pounds for the 10" drum, and 550 in/lbs for the 11" drums. Now I just threw out a round number, but it doesn't matter, you'll see that any number used gives a 10% increase!!!
The proportioning valve controls the amount of fluid going to the rear brakes. If you increase the braking force, you will alter the balance. I noticed this immediately after I changed to 11 3/4" front rotors. Prior to the rotors my car ALWAYS locked the rears before the fronts, with the 11 3/4" fronts and 10" rears it's much better balanced. Also note that the 11 3/4" rotors still use the same size pads, the increased braking force comes solely from the larger rotor. The same thing applies for the drums. For the same amount of pedal effort, the same amount of fluid pushed, you will have a larger braking force in the rear.
While it won't matter much on the street, you are also spreading the heat of braking over a larger area and a larger mass as well. This means that your brakes will heat up slower than with the smaller drums. The larger brakes also have a bigger outer surface area, which is the key factor in heat exchange. I'd have to crunch the numbers to find out if the larger brakes would actually cool faster though, because while they will exchange heat faster they also have more mass to retain heat. So even though they exchange heat faster, there's more heat to exchange, so it may be a wash on the cooling rate. My 11" drums (not yet installed) are finned too (I don't believe all of them are, I may be wrong), adding even more surface area for heat exchange. This stuff is more relevant to road racers though...
So, 11" brakes will stop your car better. You get more bang out of upgrading the front because of the weight shift, the fronts do more work (braking is usually split between 70/30 and 80/20, front to rear). So upgrading the rear brakes is only improving 20-30% of your stopping power. You will need to do something different for a proportioning valve though, especially if you're running the stock size rotors up front, because you will have too much rear bias with the larger rears.
OH! And to answer your question, I haven't seen 11" backing plates at any retailers, although you can usually find a couple sets on eBay at any given time. They usually run in the neighborhood of $100, although I've seen them lower.
This may be a set for $40
http://cgi.ebay.com/ebaymotors/MOPAR-8-3-4-REAR-BACKING-PLATES-11-3-DANA-60_W0QQitemZ170311071718QQcmdZViewItemQQptZMotors_Car_Truck_Parts_Accessories?hash=item170311071718&_trksid=p4506.m20.l1116#ht_670wt_1448here's a set of 11x2.5, $166!!
http://cgi.ebay.com/ebaymotors/Mopar-NOS-B-E-body-11-Drum-Brake-Backing-Suppt-Plates_W0QQitemZ220389356433QQcmdZViewItemQQptZMotors_Car_Truck_Parts_Accessories?hash=item220389356433&_trksid=p4506.c0.m245&_trkparms=72%3A1205%7C65%3A12%7C39%3A1%7C240%3A1318#ht_1055wt_1448and here's one advertised as 11x2", note the smaller width...
http://cgi.ebay.com/ebaymotors/Mopar-Rear-Brake-Backing-plates-8-75-Dodge-8-3-4-11_W0QQitemZ200177336431QQcmdZViewItemQQptZMotors_Car_Truck_Parts_Accessories?hash=item200177336431#ht_500wt_1463I'm not an expert at this stuff, I bought a complete set-up for 11x2.5" brakes (drums, plates and adjusters) from a fellow mopar guy. I'm not sure if the backing plates are different for the 11x2 and 11x2.5" brakes, although I wouldn't be surprised if they were different...
