I'm running a 18/39 with 19" M/T slick on my 421 cub as cast setup for race gas (C12). I did a 6.73s pass @ 104mph this summer on stock unmodified frame and the track didn't stick well.
Swingarm length, suspension, wheelie bar or not, clutch tuning...every factor is important. Like Zilla said, buy a couple sprockets and make your own test. Every bike is different...