Perhaps we need to clarify what kind of a plane this is. Is this a pusher zagi/jet were you want a direct drive, or is this a 3D foamy where you need a front mount.

The advise they are giving you is for a front mount 3D style foamy, you need to put in a gear box to reduce the rotation so you can put on a large prop. GWS and Cobri and MPI make gear reduction sets. (5.33 to 1)

If this is a jet pusher direct drive I would start with a 5 X 3 and measure the amps.