Would you be easier (& quicker?) to find a running version (any shaft length) of the one on there now, and swap the middle legs over? You'd be surprised how long some of these bits were carried across to "new" models - My 1993 Suz shares parts with the current DF30! You'd need a microfiche to suss out if it would fit or not.
This would then solve a lot of mucking around & adding weight to the hull.
Even easier - find a short shaft engine?
(But I guess you have one lined up already - in which case could you find a siezed short shaft version & swap the bits?