I'm curious what the kpa manifold pressure reads in 4th gear 3000RPM on flat stretch of road? It's important to understand where in the ignition table this steady state driving lands. I think you may actually benefit from more timing ignition in that area of the ignition table.
Here's an excellent well written book on tuning, probably the best book ever written. The author Greg Banish was my tuning mentor when I developed my MAF system for the 84-89 3.2L
https://www.amazon.com/Engine-Management-Advanced-Greg-Banish/dp/1932494421
After all the work you have done so far you will really appreciate this book.
It goes into great details on dyno tuning part throttle maps on a load bearing dyno like a Mustang or DynoPak dyno.
Nice job so far on this project!