Good advise Charlie, I'll add...you dont need a 39 trans to get the good gear sets. 39 is the last year of the floor shift, then 3 on the tree side shift boxes after that. Gonna be hard to find an actual 39 trans. What you want is a 37& up case, marked "78" on the back bottom side. This will accept the gear sets from all the 40-48 side shift trans and give you the best parts for old ford trannys. Then, you want to look for a top cover with a double detent spring set up. A screw about the size of a dime on both side of the shift tower.
Theres more details, but that will get you started. Go to Van
pelts sales, look at the info there, buy his ****** book, and you'll be all set