RE: Electrifying a great planes extra300 .46 size
You're going to want at least 125 watts/lb for an Extra. So you need a motor/esc/batt combo that puts out 750 watts. Next, I'm assuming you want power not speed for this type of bird, so you want a lower Kv motor that will give good vertical performance. Download DriveCalc (freeware) and have fun playing with the numbers. With EP there's always more than one way to achieve a desired end.
Walt