I have a Soma quill adapter with a 40mm threadless stem which allows me to use a Jones Loop Bar on my 1985 MB-2, and it works great. I chose the Soma because it was the adapter that I could find with the greatest length to get the bars way up high.
And like you, I’m in the process of putting drop bars on my long top tube MIT Atlantis. So far I’ve got the 50mm Nitto Technomic mated to a Nitto Noodle bar. I haven’t mounted it yet, but if the 50mm stem turns out to be too long, I can always poach the Soma adapter to go shorter. You can find plenty of 30 and 40mm threadless stems online, but if you need to do a 0mm stem, looks like the threaded Analog quill stem might be your only option. Having a threaded-to-threadless adapter with a threadless stem isn’t nearly as pretty as the Analog stems, but it is about 1/2 the price. Good luck with your build and keep us posted!
~Mark
Raleigh, NC