Funny, my 1980 was never a problem, except 4th gear was basically overdrive so you had to be about 35 mph. I just ran the stock tires, though.
It actually worked pretty good for towing my camper trailer. I could shift to 3rd on a hill and roar up the hill at 60 mph. I even towed my Cat on a heavy rented trailer over a high mountain pass. It went up just fine (2nd gear about 30-35 mph) but not so fun going down the other side. Didn't do
that again.
Oh yeah, that pickup got 18 mpg at then legal 55 mph, and still 15 mpg at 80 mph
Was a bunch better than my 73 that hardly got 10 mpg.