The HO52 and HO72 will look the same, there are a lot of opinions on how to tell which is which including the pinion preload bolt but most of the concensus leads to the size of the brake drums themselves as the determining factor.
Per your rear shoes, they are the rivited instead of bonded type, they can be re shoed if nessasary, Napa can do it.
Heres a couple threads for your reading pleasure.
http://67-72chevytrucks.com/vboard/s...d.php?t=200268
http://67-72chevytrucks.com/vboard/s...d.php?t=177514
http://67-72chevytrucks.com/vboard/s...d.php?t=595571