Manual Transmission Swaps

[69GMCLonghorn]

My 1-Ton Longhorn has a somewhat built 350 and an SM465 4-speed. Great set up, but I'm tired of taching at 3100 at 70 MPH, especially with good old So Cal gas now hitting over $3.25 a gallon!!! I'd like to get a 6-speed truck tranny, but I'm seeing that they are major bucks and hard to come by in "used" fashion. So now for the questions:

1) What tranny should I be looking for?
2) What will need to be done beyond using a different cross member and bell housing?
3) What about the clutch? Can I stay with a dry clutch?
4) Speedo hook up: Is there any way to use the factory speedo or will I be looking at installing a "race me" speedo?
5) Any other advise?

By the way, if anybody is thinking of responding by saying, "INSTALL AN AUTOMATIC", please save your breath. It's a truck, not a Cadillac.