Since you have the car already, you know you can measure the tray right? Not trying to be mean or anything, just want to help you as you search for batteries. To be fair the battery tray on the SC18 is an awkward size, and I come up with a practical measurement of 35mm x 105mm. It is possible to angle the battery diagonally some to possibly make it fit, but I doubt it will add another 10mm that you need. If I were you, I would cut out a piece of paper to match the measurement provided on the battery specs, then try to see how the paper fits in the chassis.

For the 6800kV motor, I would gear down to a 11T or 12T pinion at most and don't let the motor temps go above 160° if you want the motor to last a long time. I have seen motors come off the track well over 200°, but again those motors didn't last.

Best of luck!