Some comments on 4 speed vs 5 speed vs 6 speed:

4 speed: Best as a single purpose transmission. It can be set up for drag (high numerical gear), street (medium numerical gear) or road race (low numerical gear)

5 Speed: Decent multi-purpose transmission. Drag with 1'st 4 gears or road with 5'th. The compromise is in 5'th with a stout engine. Is 5'th set up as an ideal 75 MPH cruise gear? If so it is probably too tall for max top speed. Or is it set up for top speed and too short for ideal cruise?

6 Speed: 1-4 can handle drag, 5'th for top speed and 6'th for ideal cruise. The ideal manual transmission? The main drawback is size and tunnel/mount mods.