These ”velvet drive” units use a planetary unit on the back of a 71 &72 models for the reduction. They still make these today. https://www.velvetdrive.com/files/7172.pdfBou
BorgWarner marine trans had bolt on reduction units that would go on the back of the FNR part of the trans. Bought many years ago, might be hard to find. I never mounted it but a simple flat plate adapter with a bearing and seal for the input shaft would do it.