Customs 1951 Mercury Merc-o-matic questions

1i951 Mercury with the original Merc-o-matic. Transmission has developed a small leak. My questions are: what modern transmission fluid should I use and what is the proper way to check fluid level.? In park with the engine running like a modern car? Also and recomendations fora transmission stop leak?

Thanks
Les Bliss

 

you might try the guys on ford barn if you cant get help here