front disc rear drum different sizes? how to tell?

[Sdwolff98]

Just bought my wife a nice little 1981 D150 slant 6 automatic setup. Did they only have 1 size of brake rotors on these trucks? Are the rear drums likely to be 10 inch? getting ready to tear into this and get it back on the road currently runs, drives,, but no stoppie lol

Thanks
Sean Wolff

 

[Sdwolff98]

Here's the truck