4s@5Ah should give you tons of runtime and/or major power! The 2400 motor should be great on something this small. I get about 30 minutes with 2s 5Ah on a MM 4600 system geared 46/12. Granted, top speed isn't 40 mph, more like 25ish, but fine for my uses. 44/17 gearing should get you in the 40mph ballpark with your motor on 4s, and since you are using HV to get the power, runtime should be great as well, even if you do step down in capacity to reduce weight.
Mine, with stock chassis but home-made tray, mount, etc:
