There is the Borg Warner R10 OD that came out in Dodges in the early 50's and a couple years later in the Plymouths. I have a 53 model behind the 230 in my 48 Plymouth coupe. It was behind the stock 218 before I went to the 230. They are out there you just have to do some hunting. Should pretty much bolt right up to your bellhousing, the only thing may be input shaft length but you can change imput shafts. The swap into my 48 was a direct bolt in then add a couple of wires for the solenoid and the govenor and I was good to go. Made a huge difference in the way my car drove on the highway. I can run 70 all day long with no problems.
Before the R10 there was an R7 but I have zero experience with those so all I can say is do a google search.
There are guys who have gone with a T5 out of an S10 as well. That is what I had originally planned until I stumbled onto the R10. Google Paul Curtis, he makes an adaptor for the T5 to Mopar flathead swap.
Also check in over at the P15-D24 site. While it is mainly directed toward the 46-early 49 Plymouths and Dodges, there are a few guys with prewar models as well. There is a ton of knowledge on the Mopar flatheads over there.
The Mopar flatheads went basically unchanged from the early 40'as through the mid 60's. You could go with a later model 218 or better yet 230 in place of your 201 and it would be pretty much a straight bolt in with only minor tweeks.
Check out this thread for a post I did with parts sources for the Mopar,flatheads.
source for mopar flat head 6 performance parts