Either way, it looks like the mount that you have has a provision for another motor on the other side. If so, you could just use a piece of 1/4" aluminum plate to mount the esc too. Drill and tap the plate 3mmX.5mm and use motor mount screws to hold it in place. If you do that, you'll likely be able to shorten the wires, further decreasing resistance.

Maybe it's just the picture, but the shaft on that motor looks small. Is it a 5mm shaft? If this is your first conversion, that 8L will likely get you rolling, but be prepared to deal with the heat. How many cells are you going to run? An 8XL would probably be a better choice of motor.