Thanks for posting that
information and 'back story'
about the Tri-Y headers,
it was very interesting and thought provoking.

Since that design is so rare,
and because they apparently worked so well,
would you consider measuring up some sizes,
and posting them?

Things like:
External/internal primary diameters.
Distance from flange to first Y
Distance from first Y to 2nd Y
Diameter of pipe from first Y to 2nd Y