There are two. One is delivered stock on Australian cars and the other is an aftermarket unit the is similar to the Aussy one.
Both are very expensive and there are a couple of ways that you can get very clost to the same flow from the stock head with relatively inexpensive mods.
1966 Coupe 351W Top Loader rebuilt, rewired, modified for slalom and hill climb No racing - just a fun car built by and for me. Project in process
The 200 cu. in. 6 cylinder engine is a very good and reliable engine. It has 7 main bearings. Yes, you are stuck with the integrated intake manifold but there are modifications you can make to this engine. Here is a website you can check out.
I own a 66 coupe with the 200 cu. in. 6 cylinder and 3 speed manual transmission. It runs great in traffic and on the Interstate. With gas prices going up and up I like the idea that I get over 20 mpg. Also, at car shows I get many comments and questions on my six. Everyone has seen a 289/302 in a Mustang but many people didn't know or don't remember that many Mustangs were sold with a six cylinder engine. My opinion - keep the six and enjoy the car.