[jspatz1]

With the aspect ratio you describe, you should be able to turn effectively with four traction wheels if you are geared for a speed less than 10 fps and perhaps as low as 5 fps (assuming you are single speed). If necessary, plan to increase the size of the sprockets on your new wheels so that you are in this speed range. The traction and holding torque you gain on the bridge will be well worth giving up some speed on the field.