In an ideal world "B&S" is right. However, I built my last '39 box more than 25 years ago, and parts were hard to find and expensive then. Perhaps things have gotten better since then; I hope so. Probably the cheapest way to go would be to get one of the cheap generic floor shift conversions and modify it for your use. Then keep your eyes open for the parts you need. Double-detent tops and even the large (2 1/4"?) forks are getting hard to find. I sold a spare fork I had for $100 to a guy in New Zealand 10-12 years ago.
Again, hitting the old car swap meets works, and it's a lot of fun. It all depends on who you are. "B&S" is a pro with a ton of experience and lots of resoures and contacts, while I'm just an "Old Fart" that likes to play with old cars. I think that's why we approach this hobby in different ways.