Install short-to-long shaft extension – ’67 Merc 1100

I’d sure like a tip from anyone who has installed one of these short-to-long shaft extension units. I need to swap one of these units from one short shaft motor to another (better) motor. When I removed the lu with extension I saw that the diameter of the drive shaft extension joint was too big to drop through the drive shaft opening in the mid-section. To get it out I had to push it up a few inches and drop it out of the larger water tube cavity. So, I’m stumped on how to re-install this drive-shaft extension, and any tips from anyone who has done this will surely be appreciated. Many thanks, Marty

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1989 SeaRay, Merc 100HP 2+2, plus a few older boats and Mercs: Mk 58A, Merc 500, Merc 650, Merc 1000SS, Merc 1100SS (continuously learning and confused but having fun)