Ed,
Remove all bolts from leg once fin is removed, can place duct, electric tape covering holes by the moment. First thing need to do is know if OB sits at the sweet height on transom. Go for a wot spin, OB trimmed to 90? for AV plate, prop shaft to ride parallel to water level, once on plane, being a tiller OB, pull head out transom and visually check at which lower leg is water flow passing at speed. Ideal is to pass right under small upper water defector plate. Forget all about AV plate height, you'll be more confused than ever.
Assume OB is running with factory delivered prop which is ? check number on prop body. To understand how pitches works will need to install a induction tach once correct OB height is matched. Check max wot rpm as usually loaded on flat glass water. Ideal is for OB to run at near or max of it's factory wot rpm range which is 6 K.
If say OB achieves just 5.5 K wot revs with an 8 pitch, will need to drop one less pitch to a 7 pitch to pull wot revs to around 6 K. IF OB over revs say 6.5 K with too much less in pitch will need to go one pitch higher. Under powered OB's on large boats must be prop maximized in order to get the full HP out of them to perform decently..
Bear in mind, that if OB runs say 5700 wot revs with X prop, will achieve slight better top end speed than if running say full 6 K, but, as there's always a but, with 6 K combo will achieve better hole shot than with 5700 rpm, at slight less top speed. Can't have best of both worlds in just one prop, for that will need a expensive variable pitch prop...
Report your findings about OB height and wot runs with tach...
Happy Boating