Check out this thread...
https://67-72chevytrucks.com/vboard/...d.php?t=766038
I believe I went through the same thing you are going through... I had a 70 c10, but it had a 5 lug rear out of a Squarebody with the larger brake drums. Series 62’s wouldn’t fit at all... ended up swapping rear brakes for a set from a 72. Still 5 lugs but smaller/different drum mounting surface. Hardest part was finding the backing plates. Everything else I bought from NAPA.
See if that helps at all...