I don't think a 15.5" prop will clear the cavitation plate. 15" or 15.25 may be the biggest. My 14 5/8 Tempest was pretty close.

Ya from what I'm reading they are worth $2000 plus, if I measure from center of prop shaft to cav plate will that measurement jive with the prop size telling me how large I can go?

Pretty much yes. Might want to measure to the underside of the torque tab zinc. I assume there is just the plate there and no tab.

Correct no tab.