I really can't comment on a 99 600. I changed the gears in my 01 600, which is a powervalve motor, not the same animal as you have. I had good luck with 23/40 gears in the 600 APV, maintained the low and mid, and gained on the top.
I believe your stock gears are 20/39, so going to 23/40 may be too much to pull.
Sorry I can't be of more help, but I'd rather not steer you in the wrong direction.
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