No problem - One more thing, though.
I presume this is a "pusher" configuration? The key to boat drives is that the strut bears the load on a thrust bearing from the rear.
If this is a conventional "pull" setup, you will most certainly have to factor in the load on the front. Most high-performance boat systems utilize a "square drive" mechanism where the flex shaft floats in a drive collet on the motor. A fixed collet is a bit more sensative in that you ahve to factor in the changing length of the flex cable. That type of hardware is very much a cottage industry, so you won't find many of those parts in a LHS or Tower Hobbies.