How reliable it will be is all in the condition of the truck and there are multitudes of variables there that no one here on the web could know or advise you as such. But being just shy of 30 years old and from the rust belt, unless it was/is a well kept, super maintained garage queen, I'd be Leary of its reliability for heavy towing or long distances.
As far as towing goes, you have enough chassis and suspension to tow more than the drivetrain will be happy with. I had a couple trucks with that 350 and would say anything north of 5-6klbs the truck will struggle.