My Everett-Morrison has the square flange which is connected with 8 bolts (guessing they are 3/8 or 7/16").
You can contact them at
http://www.everett-morrison.com/
EM is currently producing a new version of their car, so I can't qualify whether they have headers/sidepipes in stock for the old version, (assuming that it meets your needs), but they can probably supply you with blueprints or dimensions.
- Dan