Modding the tray would be pretty hard, as you would have to make it longer and thicker, thus having to move the transmoission, electronics back (very hard). The second alternitave is to thicken the tray, then extend it out past the cars frame. The 6900 with 3s would give you around 76,000 motor rpm, I wouldn't suggest that. Already, my cars speed is crazy, but it could have better handeling. Like Sucicideneil said, it would handle better with shorter shocks. If you want speed, i would recomend staying with a 6-cell nimh or 2s lipo and using the 7700kv motor, or castle's new 9000kv motor, all with an rpm limit of 100k. 3s or 2s, you should use all heavy duty gears, as I found out the hard way.