Hello Hein, my fellow South African.
i also currently run the 3.5 Novak system. you want to have the car geared in the 11~11.25 zone. this gearing is based around rubber tyres where the diameter would usually be in the region of 64mm. so with foams you can calculate you rollout and gearing as you wear down. you should also monitor the temps on your motor and don't want it to ever get hotter that 79degrees celsius. the syste has a thermal shutdown feature, but you should also monitor the temps with a temp gun. you will definitely be flying with the 3.5R. i watched you drive at the last Nationals. you were on fire!!!
so i take it you will be running the brushless at K90 Nats?
hope this helps