[Big Dave]

Given the fact its a flathead engine, I do'nt know if that exhaust manifold is as bad as you think. It will probably flow more than those six Briggs & Stratton cylinders will put out. Once you get it running, try drilling a hole in the manifold and see how much back pressure is being generated. I'd bet it isn't much.

Go to a hot rod show and find a car with the old Ford flathead V-8 and see what headers they used.

Also, those "saddles" on the block might make it tough to put any halfway modern headers on it.