A little more "Monday morning armchair quarterbacking".....

The owner of that engine says he was only looking for 575hp, and with a spacer under the carb it was at 567hp with the Hughes heads(560hp without).

I really think with some more testing of intakes, carbs, headers, that 575hp mark could have been exceeded without resorting to a head swap.
Granted, I wasnt there, and don't know how extensively they pursued it....... But as close as they were, I think it could have been done.

One example....... The owner said they couldn't fit any spacer on that SV manifold in the car.
But what they tried was 7hp.
Perhaps a std Victor with a nice port job and a spacer might have been a little better?

When you're building a new combo and it doesn't do what you want, sometimes you have to hunt for the missing power.

Like I said....... Just more speculating on my part.