I have a 1988 242 with the 454 mag and a weiand 177, basically a 525SC clone but with less HP. I run a
22" lab Bravo1 at 6' and experience the same issues. The Bravo1 is a stern raising prop so I don't run as much trim. I also have a 23 mirage lab that also slips on the bottom end but will allow me to run more trim at speed because it doesnt lift the stern as much. I have ran these props on 22 degree bottoms and never experienced this slippage.