This isn't related to matching up splines. This is about a bind on the flywheel coupler
Ayuh,..... Welcome Aboard,..... Ya haven't mentioned 'bout what vintage this ole stuff is,...
But, I'll venture a guess anyways,....
Back with the ole MC-1s, the Chevy based In-line motors, used a shorter input shaft, than All the other motors, 'n drives,.....
'bout an inch difference,.....
I'm guessin' yer stuffin' a drive with the now standard 9" input shaft into a transom assembly that requires the 8" input shaft,....
Or, the gimbel bearin' is massively outa alignment with the motor's drive coupler,.....