Re: Recommended rear end gear ratio for TKO 600
IF you decide to change your rear gears, I'd go to 4.10's -- I don't think the change from 3.55 to 3.73 will be significant enough for you to really feel it much.
As far as the comments about the usability of the 0.64 5th gear, I guess I'm special since I found 5th usable behind my 307 even with 2.73 rear gears. It wasn't exactly a rocket ship in 5th, but would run down the road OK at 55mph. Of course my engine was mildly cam'd and carb'd and ran a lot of spark down low to keep things running smooooth. If your engine doesn't like low RPM, you can end up with issues with the wider 4/5 spacing... (Finally changed over to 3.73's now, but am still working through re-bleeding the brakes and getting it back on the road).
From your above description, I think I'd spend some $ on cylinder heads 1st. With the 70cc chambers and 8.75 compression, you're losing some torque and efficiency. Cam is a bit mild (212/222 at 0.050" if I'm reading the specs right) and probably won't like to rev too high, but should play decent on a street motor. Maybe go a step bigger if you're OK losing some bottom end to pick up higher RPM power...