Different Manual Tranny?

[Lobo'74]

I currently have a 1979 all syncro 3 speed in my '67. With the 4.10 (4.11?) rear end I get almost 10 mpg... on a good day...

I have been thinking of a 700R4 swap in the future, but I saw a post where someone mentioned a 5 speed from a 1990 half ton. This might be more do-able in the short term. How difficult is it to swap a newer 5 speed into our trucks? Mine is a '67 with a '79 350 that has some..."improvements".

Any inputs are appreciated. Thanks.