By far, the best choice for this would be a small base Rochester 2G. Mount them backwards on the manifold (they don't care; really). Everything falls into place, throttle linkage, fuel line, and even the automatic choke. These Rochesters, like the Merc's, come in both integral and divorced choke varieties. With the right carburetor, it's a no brainer. On the integral choke models, all you have to do is shorten the choke stove tube; for the divorced choke model, you just have to bend the actuating rod slightly. I have one on the '51 Merc in my '51 Ford, and I had another one on the engine on my test stand. I have some pictures, but I think they are on my other computer in Minnesota (I'm "Marooned in Marathon" by the virus). I'll look and see what I can find.
I find they are the best choice for use on a mild street flathead (on a car with a hood). The only problem is finding one; there are a TON of large base 2G's out there but the small base models are getting hard to find.
Oh yeah; one last thing. While the carburetor will bolt directly to the Merc manifold, the throttle bores are about 1/16" bigger in diameter and the manifold bores should be opened up. I have a jig and a devices called a "Rota-Broach" to do the job. Bigger carb, more power.