I have modified an HEI for my GMC 6. The process is complex because the weights, springs and center plate shape work together and against each other to affect the advance curve. Two excellent sources I used are available on the internet free: (1) "Changing the Advance Curve on HEI Distributors" by Jim Hand and (2) "Ignition Timing for Modified Engines" by Dave Andrews.
I got my HEI to match the curve recommended by Dave Andrews and it made a great imorovement in performance and driveability. If you decide to follow Jim Hand's recommendations, go easy when you file the weights. A few (2-3)file strokes will retard the advance starting point point and widen the range by 1 degree.
