wrote
Almost 100% sure. Outdrive is a pre-alpha (1977)
But tomorrow l"ll give 3/8 a try. Thanks for your
help.

Mine's a 77 also, all original, and it's 5/16. Have you actually seen down
in the hole and confirmed that it's stripped? Something that's happened to
me a few times is I miss my target and get the wrench wedged between the
outside of the bolt head and the upper housing in front of it and it feels
like it's stripping.