Why won't my 15HP long shaft bring my aluminum boat to plane?

[honda2017]

I have a 15hp long shaft that won't run at top speed, after about 50% throttle it seems to cavitate. . . though it could also be described as drive slippage. the motor shaft is about 3 inches too long for the transom in my very old aluminum skiff. Could the slippage I feel be related to the shaft being too long?

[boats.net]

No being too long would not cause any cavitation or ventilation, check lower unit oil for metal in oil or burnt smelling, if so then probably have gear damage. Might check prop to see if hub is slipping.