what prop
We need some more information. What is the plane? Has it flown before?
If the first flight, I would prop for thrust to get out of any troubles on takeoff and to limit top end, so 10x6 or 10x7. If a quickie or dust, I would try a 9x8 or 9x9, perhaps a 9x7. You have to see if the 8x10 has enough thrust for your application (ie a sleek airframe). When all else fails, my speed prop of choice for .46 to .50 engines, is the 9x8, and I experiment from there.
Paul