1979 Chevrolet Malibu 496-TH400-9" (cruiser). 1992 Chevrolet S10 355-700r4-7.625" (daily driver).
Have you checked the total timing on this set up? Figuring out where it is at various RPM may provide some benefit.18-20 initial, vac adv disconnected .. Hei stock.
When you have some free time do a google search on "recurving HEI distributor" and "limiting vacuum advance." It would take pages to explain the whole thing. What I can say is the way it is now can certainly be improved. I think getting the mechanical advance in a little earlier than the typically slow advance of the stock HEI and operating with an adjustable vacuum advance should be an improvement at part throttle and overall.