It is possible I shortchanged the GT.

I was actually using some of the data from your book on the GT exhaust and the S manifold.

The closest I could interpolate was your 428 T bird engine with the GT exhaust, using a decent intake, and then the same engine with the S intake and the dyno headers.

Taking both, deducting a bit for the 40 ci less displacement, results in a pretty low hp unit. It may be more than the 275, but I realize that few shops actually old stock engines!

I agree that waking up the 390 is certainly a case of intake and exhaust.

The old solid lifter 330 hp unit that a friends 63 Galaxie Fastback had is a far cry from the stock GT. It had the good iron manifolds and C3 Iron Intake.
So based upon your test data, and that engine, is why I figure the 390 GT was pretty anemic.

50 hp or so is on the table, with simple changes to a 390. I was amazed how even the Performer 390 was such an improvement over the "S", and using FPA headers had to bring you to 325 hp or so.

That engine is pretty much covered in Jay's book, more or less.
400 hp+-, maybe closer to 375.

Those "w's in the curve!!!  Uggh.

The run seems to start dying at 4000, like it has a stock cam, and except for the later "w"s, you'd assume it was so. W's= something wrong for sure.
The torque peak was certainly pretty much correct.

I remember from the video that there is a bit of clanking I can hear, like a loose rocker??

The torque produced is as you say, about what a healthy chevy should produce, so the dyno operator should have a good feel for things.

Should have been nearly 400 hp for sure.