It looks like it has a curb weight
of about 3200lbs and a GVW
of between 4200 and 4800lbs.
A 77 Overlander has a tongue weight of 580lbs so it looks good to go.
Keep in mind that GVW is based on 1953
tire technology that would put it on the conservative side if new load range D to E tires are added. High load tires and upgraded springs should add a substantial safety margin. Lower gears in the rear axle will bump the towing capacity.
I am no expert on old trucks but you may not be able to find "towing capacity". That truck was built in the day when people used common sense and just hooked up things safely. Towing capacities that everyone frets over so much these days are half math and half legal CYA.
I worked in a marina in college that used a 1960
1/2 ton Chevy with a small block v8 and a 2 speed automatic to tow some massive boats. It saw 8,000 - 10,000 lbs on a regular basis with no WD or anti sway. Ah - the good old days