The 40 works great! I am running 70cc aluminum polini with polini case matched, doppler clutch pulley and variator and exhaust. Using 2 heavy weights and a medium on the variator. It revs to pretty high rpm and then shifts. you never feel any loss or gain of power as it shifts. it pulls all the way through and hard.
a 50lb gas strut would work well too. might even start easy without a decomp valve. You might have to use 3 heavys on the variator. maybe more. When i was running two springs I had to find longer screws to stack up the weights on the variator to get it to variate at all.