In searching for info about flatheads, I ran across this post. The manifolds you are talking about are on the big 30" head motor, it comes in 3 and 4 ton trucks. Totally different motor, not many around, and pretty big and heavy. There may have been a very few on the smaller 25" motor, but I have not seen them.
Tom Langdon of Landon Stovebolts makes a pretty darn nice tube header for the motor you have. Comes out with the 3 front on one header, the rear 3 on the other. Cost in the $300 range. I have one he made for me recently and it's top quality.
