I have the HiMaxx gearbox that is for 20 mm brushless motors, it's blue in color. (You can get this at
www.AeroMicro.com for $24.95) The motor slides into the housing, which is also a heatsink. (that just works excellent!!! motor now stays at least 20 deg cooler) It comes w/ three different gear ratios, however, I have found the 6.6:1 w/ the E-Flight 12x6 prop THE BEST setup! (Order more 66T spur gears for around $5 apiece) The GWS shaft you have will work, but I'd get a few extra HiMax for only $3.00 apiece. 3x60 mm threaded.
I also tried an APC 11x7 for more ground clearance, but it just didn't climb out the same.
I checked my amp draw w/ an amp clamp and was around 18-20 amps depending on charge, with that particular setup. Trust me, just as Jason Merkle says, the 12x6 @ 6.6:1 is the best bet for powering this puppy. Mine now does AWESOME waterfalls, in fact, just about as many as you like in a row...........way cool! I have Futaba 3108 servos (17 oz ins) w/ HiTec arms that I made longer with their adjustable arms, which gives me Lots of throw, probably around 60 deg up and down.
This plane is sooooo much fun, much like my 40 size. Elevators are killer, you will need lots of throw!
Scott