Propeller or First 25

I sail a First 25 lifting keel and replaced my engine (volvo 7.5 @ 2600rpm)last year with a BETA 13.5 @ 3500rpm and also replaced the prop with a 12 x 8. I am getting severe cavitation at all revs above 2000 which appears to suggest the pitch is incorrect. Can anybody state what size prop is best in practice?
A&T