An 8x4 is a bit large for the engine, but not out of the question. Try an 8x3 master airscrew prop. We also used 7x4's for combat, so maybe a 7x4 or 7x5. I think 7x6 might also be a hair on the big side.

The Grish props, if I recall, are very fleixble, right? You might also try an 8x4 master airscrew or another brand if the Grish doesn't work well. Different brands of props in the same size can be radically different, espeically in this size range.

How much does the plane weigh, and what is the wing area?