Paul-
Why not try advancing the timing to 10deg btdc static and see if that helps. Pay attention to the possibility of Knocking/pinking. This would be most noticeable under load. If it doesn't help, change it back. When setting up a car I have under similar circumstances I used a testing method as follows:
1. Warm up engine to operating temperature.
2. Disconnect and plug vacuum advance.
3. Set timing slightly advanced from stock.
4. Make strong acceleration run under load listening for pinking/knocking.
5. Advance timing until very mild pinking/knocking.
6. Back off one or two degrees.
7. Reattach vacuum advance and test drive.
Hope this works.
Also, it could be that after you added all the performance goodies your carb needle is too lean. I wonder if you could take the car out at highway speed and watch it overheat slightly and try pulling out the choke a little to enrich the mixture slightly to see if the problem goes away. I'm not sure this is a valid test but it seems logical.
LCJUTILA