79 to 71-72 rear brake backing plate swap???

[MP33]

Need to know if the 79 C-10 backing plates that I have will work to replace the rusted/shot 71-72 backing plates. I just have the backing plates on the 79 rearend, no drums,shoes or anything else. How do I determine if they are 2" like my 71-72 or the heavy duty brakes? I doesn't matter to me if they are the heavy duty or 2" but I need to know what to buy tomorrow. Thanks.....Mike