I agree with Sal, I have had good results with rebuilding the stock two barrels on 49-53 Mercuries. The carburetors look strange when first opening one up, but if you follow all the passages and parts they have all the normal carburetor functions and circuits (idle, main, power valve, etc.). For me methodical cleaning, making sure surfaces are flat, and assembly with all the correct parts has always resulted in a good running carburetor.
I recently had my first experience with a "teapot" four barrel on a friend's 56 Ford. It was more of the same, with good result following the same process. For folks who must have a four barrel on their flathead one of these four barrels with careful rebuilding should work well, probably one from a 54 Mercury would be best?