Slant 6 with 4 speed

Wow! Thanks for all of the excellent info.
I saw a duster slant six manual for sale and was dreaming of swapping out the trans to 4 on the floor to make a hotter performer with gas sipping at cruise speeds. I now see that there is a bit more work to be done with gears for what I was thinking. Would a 8 3/4 rear with 393 gears do the trick? Or would gears in the transmission also need to be changed?
Thanks again for sharing your knowledge.
B