I don't the answer myself but I suggest you join
www.roadfly.com and ask your question there. The transmission expert is named Kirt Koeler. You can search the archives or just browse through the listings to make contact with him.
Another guy, Gale, recently converted his car to a manual transmission so I think he knows what he's doing. A link to his tips page is here:
http://www.nmia.com/~dgnrg/page_2.htm.
Good luck and thanks very much to your son for his service.