An 18 x 6 is commonly used, and works well. If your Ryobi is modified, it might turn the 18 x 6 too fast. (7500 rpms max static is what is recommended for the Ryobi.) In that case, you could go to a 20 x 6 if you have the clearance for one. If not, you will have to use an 18 x 8, but an 8 pitch won't slow the plane down like you want...

AV8TOR