14 cells will do the job, 12 is a bit too low IMO. Get the 8XL if you want a rather long runtime. The 2 speed transmission will keep the motor nice and cool..
The GP3300's are the cells i am still using. They deliver terrific punch.
40-45mph is asking a bit. Get the 7XL for that, but you will sacrafice runtime.
Check
this calculator to do your math.