Slick 60's

have a 63 stepside with 289 3 speed and 2.75 rear gears what is the towing ability of my truck would like to tow a vintage boat or camper like airstream of something similar thks

That is not a good towing gear. You will not have a bunch of power from start. How heavy of airstream?

agreed 2.75 not the best gear for towing but how many big cars had those gears and pulled boats and trailers.

stay in the flatlands and you will probably be ok, but will stay in 1 and 2 much longer getting started.
any big hills and you will be climbing in lower gears.