Ok,... I was wrong, the retro coupler goes the other way, newer motor to older drive,....
Best I can figure, yer gonna have to use the Merc donut coupler for the 2 pc. rear seal motor,.... 18-2171,...
OMC came out with the Cobra the same time GM went to the 1 pc. seal, so there's no Cobra coupler for an older motor,...