6.3:1? I wish I had that much gear to play with, I have 8.5/10:1 2 speed in my old beast!
However, with the 1:1 4th gear ratio, even at 8.5:1 I can run up to 60 if needed & it's pretty comfortable trundling along at 50-55. I would think that if your engine can push the RPMs you could run the same speeds. If not, adding more gear is just gonna make it into a pig that takes forever to get up to speed & cant climb hills without down shifting. Like Berry said, without an engine upgrade those gears wont be much help.
Oh, & dont forget your matching brake system upgrade! Going is optional, stopping is mandatory!
