I am just beginning to build my P-51 and have decided to up grade the motor to a GWS EPS 400C with a gear ratio of 3.90. I have also decided to go with the 1080X4 and a 3 Cell 11.1V, 1500 mAh Li-Poly Battery. Does anyone have any do's and don'ts
for me or is this a bad idea all around? I am also going with retractable landing gear. I am really looking to get some speed out of this thing without killing it the first time out. Any help would be welcome.

you'll probably want to fix the timing too... from what others have concluded, the motor has about -4^ of timing, so you may want to add 4-8^ or so... This should wake up the motor some.

The 4 blade prop has a reputation of being garbage as far as performance, I'd stick to a two blade setup, and maybe experiment with the 4 blade a little later...

Phreakish what do you mean by timing? the gears on the reduction? I will take your advise on the 4 bladed prop. What do you think about batterys for best proformance and flight time for the flight time.
Simituff

well, I love kokam lipos, 1500hd is what I use but the 350 only draws about 8A (12A max).

Timing meaning the timing of the brushes in relation to the commutator. You can rotate the back plate of the motor to get more rpms, at the cost of amps (until the law of diminishing returns kicks in).

I just got through with another flight on my stang. I added a couple degrees of timing to MY 350, and it flies wonderfully. It wont hover, but it'll torque roll at the top of a good climb (about 75 ft). Its fast, not lightining fast, but good for a parkflier this size. I'll get some video for you.

I havn't tried the 400 yet, so I can't suggest much on that, but if you do use the GWS motor, it has to spin in reverse rotation to spin the gb the right way, so it's timing is retarded (relative to reverse rotation), so you have to add some to it. Search for some threads about retiming, and a thread in power systems called "GWS speed 400 needs timing?" or something to that effect.