Apart from the 3-port design, there's not much wrong with the Dodge engine...
If you were really adventurous you could see about cutting the port walls out and welding in new walls to separate them.
Others to check... Pontiac, the Hudson 308 (early fifties), Oldsmobile, not many really. Did Packard make a six?
The only flathead 6 I know had seven mains was the ZSB Chrysler 6 from the twenties. On the other hand, the only flathead 6 I know had six inlet ports was the Terraplane from the thirties, which was very popular in racing here.
But you might not like the lube arrangements on it... to keep oil up to the dippers the racing people here had a little oil pump pushing pressure into a tube mounted inside the crankcase, there were holes in the tube to squirt oil in their direction.
