help on how to measure for a driveshaft

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Hi, I am missing most of my stock 144 mm pump. I have only the tunnel piece on the outside and the 67E midshaft on the inside. i can get the parts but I'm told there are various driveshaft lengths depending on midshaft set up. I have enclosed 2 pics of my midshaft. First shows whole thing in place; second shows midshaft removed leaving piece(which says 65A on it) that bolts to the bulkhead(what is that piece?) This yamaha jet ski type driveline is in a 10' jet boat.Only other point of reference I have is the 4 holes in the hull where the parts of the pump bolt up. 3rd pic show another boat with pump installed.

How do I measure for the driveshaft length?

or

does anyone know what length driveshaft would be used.

Thanks