The robot will want to turn about its centre of mass. Ignoring spinners which mess with this, you want to get your weight as far forwards as you can. That is the simplest option.

The issues with messing with the wheels are that A) the banebots wheels are 73mm and the robochallange wheels are 75mm which will mean that one set is trying to drive faster than the other. This can be solved by lathing down the robochallange wheels to match and b) the banbots wheels will wear down faster meaning in order to keep the wheels the same size you will have to keep turning down your rears. A few mm may not seem like much but in time it will kill your motors.

To give you an idea, in 4 matches Conker 3 went from having 5mm of ground clearance to having just 2mm and Binky went from 6 to 3. Both run Blue 73mm banebots.

Just some additional info for you :-)