The shaft should slide out the back, not fwd into the cabin.  Also, neither the motor or prop have to be put in the centerline.  I had a 32' sharpie ketch with offset 30 hp Atomic 4 engine and centerline shaft/prop.  The motor wouldn't fit on centerline because of the mizzen mast. It was chain driven just like a bicycle and worked fine.  Another 42' gaff ketch had both 60 hp Chris Craft engine and prop off center.  Built new back in 1934 this way.  It isn't the most efficient setup but I couldn't tell the difference when motoring. 

Bill P.