I'd expect towing info to be in the owner's manual.
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Thanks a lot guys, lol yea the bed looks short to me. Maybe because its all beat up...Take a look here: https://www.xr793.com/wp-content/uploads/2019/06/1990-GMC-Sierra-R.pdf
I don't see towing specs. They do list GCWR based on axle ratio... so figure out what your truck weighs as it sits and subtract that from the GCWR and do your other checks like pin weight, etc.
For example, say you have the 4.10 axle and a 5.7 gas engine (L05), the GCWR is listed as 13,500 lb on page 15 of that article. If your truck weighs 5,000 lbs, then:
GCWR - truck weight = towing capacity
13,500 - 5,000 = 8,500 lbs
...pretty close to what that other source lists. I just made up the 5,000 lb truck weight, but it's gotta be a close number?
Good luck.