I think that with the classic engines, the only advantage a 6th speed gear can do is to allow a greater top speed. Cruising at 70 in 6th, an rpm of 1400 with a lumpy cam??? Soggy throttle response, fouled plugs, worse mileage...
I'm going with a 5 speed setup. The gear ratios for the overdrives are often 5th: .75 and 6th: .50
Depends on the 6spd of choice. I have the close ratio T56 magnum that runs a 0.63 6th gear, I run about 70mph at 2,000rpm with 3.73s. High enough to not lug it constantly but low enough to run down the Texas highways at 80-85 while under 2,500rpm.
Here's my trans vs the old 1:1 904
