What was the top speed of a 1965 3/4 ton truck with the stock motor and transmission?

My uncle recently purchased a 1965 ¾ ton truck, and it reportedly has a school bus transmission in it – and a top speed of roughly 50 mph. You don’t use first gear at all on the road with the bus transmission – you start in second. It’s a three-speed transmission, but first is like a “granny low” gear.

What transmissions would go in a 3/4 ton truck and would a strock transmission give better speed?