Jammin X-1 CRT motor selection

I am in the process of converting a X-1 CRT Pro to brushless. I eventually want this thing to be sick fast and would like to run 6s once the Monster Max comes out. I am planning on getting one of the less expensive motors and running if from my existing Mamba Max on 4s, then upgrading whenever the Monster combo comes out.

What motor is best for me and where can I get one? I have done the motor selection tool and top speed calculator and it looks like the 6XL or 7XL will work the best. Mike recommended a 8XL also. I am stumped on which motor to get that will give me about 50 mph and 20-30 min of runtime. I am looking at getting a set of maxamps 4s 4000 packs and running 2P. Is this a realistic expectation to get these speeds and runtimes?

Anyone have any suggestions on which (or another) would work best? I will not be racing, but just going fast and taking on big hills and jumps.