good tranny to change to?

[Gus63]

I have a 63 chevy k10 pickup right now with the factory sm420 tranny in it with a divorced transfer case. i would like to change to a different transmission, either a 4 speed without the granny, or a 5 speed. i basically want to be able to cruise on the interstate without running 4500-5000 rpm's. what are some good transmissions for a truck that could be put into my truck without much modifying? i still need to be able to tow when i need to, but not much. i was thinking about a nv4500, but they are a little too expensive for my budget.