^ Interesting. I'm not a drag guy but use mine on oval track and focus the clutch tune on the hole shot, and it happens to slip a bit between gears getting out of the turns.
Basically it becomes a fine combo of gearing/tire, spring pressure, and weight. I went through plenty of springs and tried lots of different weight.
So you're saying it doesn't have enough weight to fully lock the clutch up beyond the 60'? Since you say it 60's ok, but still slips, that means the current springs can slip the shift like you want. So try giving it a touch of weight till it just locks up all the way. Then prepare to sacrifice some springs from there.