18x6 at 6k RPM is about right. Smaller prop will increase engine speed but tends to reduce thrust. Engine will pick up RPM in air as you unload the prop. Lots of info on here about getting set up with gasser engine. Standard mods are carb, intake, and exhaust porting. Depending on engine weed wacker can go from mild to wild your choice. Spark advance is not a big issue especially at high RPM. Charge turbulence takes over at high speedI like a large prop at moderate speed for a nice relaxing day at the field.