I already tried using more initial as well as limiting the mech advance and locking it out. I couldn't tell any real difference in how the car drove, so I reverted to the set-up I have now (with full manifold vacuum advance hooked-up for regular driving and crisp off-idle, part-throttle driving).
I just this second looked at the graphs and annotations from the chassis dyno and I must apologise as I got the figure wrong... it's not as bad as my memory suggested... total timing was measured on their timing light as 34* @ 3,000rpm and 43* by 5,800rpm. We tried retarding it to be 34* @ 5,800rpm and lost power (only a handful of HP, but still a loss). Whether this was due to my restrictive exhaust causing all sorts of issues, I won't know until the new system is back from the fabricators.
So, you're saying with a stock HEI (even one that's been recurved with lighter springs), the timing will never truly be all-in by 2,500-3,000rpm? This creep will always happen?
Yes, I tried using the lightest springs and found it was adding advance even at idle, so immediately went back to the medium weight springs, which made the timing APPEAR to be all-in by 3,000rpm.