The stock 4-Speed trans for that truck would be a SM420, which is either very similar or exactly like a school bus trans. Unless very specifically verified, it could very well be the original trans in there. Many don't use the Granny Gear on the road, like I do. Changing the trans won't give it any extra speed as 4th gear will still be 1 to 1, unless you go with an overdrive. With what you have, what would make it go faster at a lower RPM, would be to change the gears in the rear-end.
Bill
