Looks like you've got some research to do.
I don't know what your truck is but I'm going to assume 1/2t.
Western lists separate part numbers for the 73-87 C10
1" rotor - 2 1/2" drop (part # 13200)
1 1/4" rotor - 2 1/2" drop (part # 13100)
There is also a different part number for 88-91 C1500 - 2" drop spindles. (part # 12400)
The C3500 series have different spindles also.
73-87 - 3" drop (part # 13600)
89-99 - 2" drop (part # 13700)
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I see 4 different applications for calipers on 2wd 1/2t pickups 73-87.
73-78
79-80
plus 81-87 have 2 different caliper apps depending on rear brake size
88-91 have 3 different caliper apps depending on the production option.
Clear as mud?
Sorry I couldn't be more help but you are going to have to get more info on what you actually have in hand.
Casting/part numbers on your spindle and a call to Western Chassis (Tech 888-999-9139) may give you the info you need.
Remember, Google is your friend. Major auto parts sites are great for finding what stock part numbers interchange with a little time searching.