I know absolute zero about boat motors, but a friend bought the twin 413s out of a burnt boat, (never used them, one counter rotated tho) and it seems to me that they had a different rear flange, maybe like an industrial engine? I haven't seen them in years, but he probably still has them somewhere, never parts with anything ...

I suppose it is possible that it was just an old boat, and they were pre 62 style cranks.

It may be that newer boat engines use the standard flange though.

Just something for you to be sure of before buying.