4 speed manual transmission upgrade to 5 or 6 speed
 

I have a 69 resto mod with a 671 on a sm blk 350. I currently have a Muncie 4 speed transmission that runs around 4K at 65mph. I like driving it to shows and was wondering if I could drop a 5 or 6 speed manual transmission in, how far could I expect the RPM's to drop at cruising speed (65mph). I was also wondering if there were any advantages to going with a 5 over a 6 speed and visa vera.