Flickering Lights

Couple comments. Is this "flickering" ---is it cycling up and down IE you hold the throttle at a set speed off idle, steady, do the lights "cycle" up/ down /up etc? Or are you referring to just dimming down at idle?
With all that you've done, you need to re-check for voltage drop to the regulator, and if that is OK then as suggested get a solid state regulator replacement.

If you add just one wire--since you have a squareback---you can convert to the 70/ later regulator