They probably didn't list towing capacity for anything other then the 1 ton trucks back in the day those trucks were new.

3500 towing and 1500 weight capacity was probably about right for a d150.

I pulled a 3200 lbs dirt track car on a 1,000 lbs trailer, with probably 1,000 lbs of tools, jack, & tires in the box. On nights we were on the way home from a rain out, it was a pretty white knuckle drive. My big block truck was set up to tow and it was a very good trailer with brakes. It had the power, but I was on the edge of the suspension, steering, and brakes, even with the trailer brakes. One season was enough.

The 3/4 ton I replaced the 1/2 ton with about a year later was much better. Several years after that I replaced the 3/4 ton with a single wheel one ton, the loaded trailer didn't effect it at all. Gene