Ok, well, if you want a little more top end, you'd go with the 60. If you want more power, you'd go with the 70.
According to the
calculator with the 60 @ 3500RPM you could get up to 33MPH
With the 70 you could get up to 29 @ 3500.
If the total weight is around 250, I think the 60 would do ok. I have mine set at a really low ratio, <5 actually, I don't remember the exact number as my axle sprocket is an odd number. I weight about 170 Plus the kart, which is quite heavy. I think total it would surpass the 250 mark. It still picks up pretty good. The motor is a bit bigger though at 9HP, but I think you should be ok with what you have.