Curt C.,
Even in the 306 test the regular GT-40 heads (not the GT-40X) made more torque than the AFR165's. Find a direct comparison with the GT-40X and the AFT165 and I'll bet $20 the 40's will make more torque.
Again, it's not about HP... TORQUE RULES!
you said,
"...but that still does not mean the GT40X head belongs on a Supr427's 351W much less a 408W."
Did I miss something? Where did I imply the GT-40X head belonged on a Supr427 351 or a 408?
For a large displacement 351, I would go with the Z304, AFR185's, or Ede Vic Jr.'s. The AFR205's would be a nice race head - high RPM motor. Just my opinion, but what do I know, I'm selling all my sbf stuff and going FE!