Firstly, inflate sib's tubes and keel to it's max recommended working pressure, that's with a gauge. Float sib alone, trim engine to be perpendicular (90º) to water surface, sit on middle deck as forward as possible, go full wot, check if with over or outter transom water splashes. If so engines needs to be height corrected on transom, sits too low. If still cavitates, aireates your engine is way too high on transom, bad news will need to chop transom down..
Bear in mind that a 4 strokes 6 HP engine is much heavier than same 2 strokes brother, that's why you need to sit more up front than with previous engine to compensate heavier engine weight.
To play with pitch sizes will need to install an induction tach to check max wot rpm with you alone and lightly loaded sib. Try to achieve this "sweet height" engine/transom condition by placing wooden shims untill correct position is achieved.