First suggestion, add at least 3" to your prop diameter and 3" to the pitch. A Speed 480 should easily be able to handle a 7x3 prop direct drive. With a 4.1:1 gear ratio, I'm almost thinking as large as 12x8.

How are those Speed 400s holding up on 3S LiPoly, especially with those big 6" props? I'm betting you probably don't get all that many flights out of them. If you don't mind changing them, that's okay.