Weight is definatley the key for the M-03 I think. I used to fill the back of mine with lead, make sure it is low down as it will stop the car from rolling over. Back bit where the servo for a mechanical speedo should sit the best place to put it in.
Only other thing I used to do to mine was turn the shocks upside down as it lets the car sit lower for some reason.
Good luck with getting an m-chassis to run straight, the early ones always tended to be a bit wanderey.
