I checked some magazine article scans on the web. (
http://www.thunderbirds.org.au/images_combatplans)
On the Demon, the engine mount ends right at the start of the wing spars.
On the Voodoo, the engine mount ends well before the wing spar.
On the Scorpion, the engine mounts overlap the wing spars.
All three have the 2 hardwood engine mounts connected vertically with wood, either balsa or pine.
All three have the 2 hardwood engine mounts + connecting wood sandwiched between two center ribs.
Hope that helps, at least the web site will give you additional views.
-Chris