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