Try taking an old stock motor, mounting it and the heaviest set of wheels/tires you can find. Charge up a set of cells and hook them up directly to the motor, letting it run until the motor won't turn anymore. Keep the car suspended with the tires off the ground.
Repeat as necessary, until the drive train is broken in until your liking, I personnally bolt up my Aluminum set-up wheels because they are much heavier than any set of wheels I own.