Help with 1955 1st Chevy Rebuild

[Boxie]

Hey guys,
I am new here so forgive me if I ask a lot or ask stupid questions. I recently received my great grandpa's 1955 1st pickup that he bought new (a dealers demo) in 1955. I have been looking to make sure everything matches but can't get a fix on the transmission. My family says it has never been worked on but the numbers may say different. It is a 3 on the tree and the serial number on the trans is M8C18. That to me translates to a Muncie built on March 18, 19?8. Any help would be greatly appreciated.