cavitation plate height

I have 14ft broom gemini speedboat(UK)
It is rated for a short shaft outboard.I have raised the transom height so I can use a 25hp johnson longshaft.
Unfortuately my calculations were out,and now the cavitation plate is 3" lower than the bottom of the stern/ hull.(using the 25hp Johnson longshaft)
The boat has been converted for sea fishing,so speed is not an issue.
Will I get away with leaving it as it is? The boat has a splashwell on the stern.
The Johnson is a 1999/2000 25hp model (two cylinder)
The boat has a pointed bow and has an almost flat stern. (very stable)
I don't want the expense of a jackplate.

 

The plate is there to prevent ventilation, not cavitation. When it is that low, it will create more drag. You probably need to add a higher extension.