1970 GMC 1/2 to Tranmission question

[2guns]

I have a 1970 GMC 1/2 ton with 4 on the floor, first gear is very low start off in 2ed. 4th gear is a bit too low for me top speed is 60 it can go faster but the rpm is too hig for my taste.. would like to know if there is another trans i can drop in that can give me more of a freeway range? i do not go fast but i would like to be able to cruz at 65 to 70 without winding out the engine. it has a 292 in it and i have no plans to change that i like getting 15 mpg, thx JD