You won't find any published data, because I don't think they did that back then.. Maybe GVWR at most?
At this point in it's life, it weighs what it weighs, so drag it over some scales since you have it hitched up and as long as you are comfy towing it, it should all be good.
Take a look at the tire weight ratings.. At least that will give you an idea of how much they can handle. My guess would be that your axle ratings are probably #2500-#3000 each.
Figure on not going much over #5000 and you should be okay. Everything is over 35 years old now, so gotta take that into consideration too..
Good luck! Mitch