Can not see what you are pointing too in the photo. If you are talking about the piecer spot welded to the backing plate under the anchor pin at the top that the springs attach to: then yes if missing it will affect the line of force applied by the wheel cylinder, as it expands expecting the shoe to ride on that double thick portion of the backing plate.
Backing plates are interchangeable over a wide number of vehicles from GM. I would find out the Hollander interchange number and look for a used replacement (they are not reproduced). You could also go for a wider two and a half inch wide shoe and drum with different a backing plate off of a light truck. Once again all bolt up to the same four bolts used on the Spicer axle flange and you can find five bolt 2.5" shoes to fit off of a Police vehicle or a Station Wagon. Or you could redrill a six lug drum off of a pick from 1965 to fit your smaller five bolt pattern.
GM was run by accountants not designers or engineers. They didn't put anything on the car that could possibly left off or made cheaper. Chevrolet as the lowest cost entry level vehicle was the stingiest of all of the marquees. If there was a tab welded to the backing plate you can bet your Bippee that it was there for a good mechanical reason. I am still a little confused as to which part we are talking about.
I still have two twelve bolts out in the barn (one is actually a ten bolt Olds rear I bought by mistake as it has 12 bolts on the inspection cover but only ten on the differential), but the backing plates are pretty rusty now (and one was bent when I dropped the rear end moving it from one pile of spare parts to another). I may be able to get out there and photograph one of my backing plates and post it to let you highlight it in a photo editing software suite to mark up what is missing.
I use PhotoBucket to post BIG pictures on these sites (since I usually photograph them with a 12 mega pixel Nikon camera). It gets around the restrictions on size of photos required by this boards band width and storage allotments.
That is what I thought. The e-brake attaches to the rear shoe at this point and the rear strut then forces the front shoe into the drum activating the braking. With out those tabs the strut will not push in a straight line and could cock the brake shoes (which I am pretty sure wouldn't be good). The strut is sometimes called an expander, or activator.
If you never needed your e-brake (except to use it as a parking brake) you might be able to get away with it being that way. However I don't know how reliable they will act in the case of an actual emergency.