It's not a matter of just looking good, you should perform a visual inspection of the venting problem with the following :
-Inflate sib tubes and lower speed tubes to at least 3.0 psi with gauge.
-Trim engine to be perpendicular to sib when floating.
-Place a mate near same weight as you up front.
-Go full wot, with driver sitting on rear tube or inside deck next to transom visually check water flow passing through lower leg.
-This test must be done preferably on flat calm no wind seas, close bays, etc.
-With respect to transom, probably is intended for a long shaft engine and you have a short shaft horse or viceverse issue.
-Spotting at plane is the only way to know how much you need to lower or raise the engine, depending if having venting or transom back splash issues.
-Engine height as stated on all owners manual is just a starting refference, if wanting boating perfection engine/transom height must be adjusted/done under trial and error...
If with excessive airation, ventilation water flow must be passing slight over or under cav plate (2) worst at close turns or at choppy seas, your ideal water flow should be passing slightly under small upper plate (1) best performance under any sea conditions, superb top speed and inmaculate close turns at speed. All you need to do is distribute sib's passengers well for a perfect sib ballance and know your engine power capabilities.
PD : Your sib is intended for a long shaft engine, probably using a short one, that's why in the need to chop transom down 40 mm, will need to chop down more to be at the correct engine/transom best performance height. Try deflating completely your lower speed tubes and test that way if the problems betters a bit.