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Grounding the radiator and heater core directly helps. Also rechecking motor to body grounds.

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Radiator is grounded to frame, battery is on the same point. Battery to engine block, starter to subframe, engine block to subframe. The subframe contacts are cables, soldered into large bare terminals and then heat sunk and the exposed bolt attaching it is covered in hot glue to resist oxidation. I was pretty religious about grounds. The only thing I have not done is the heater core, which makes sense now. I can probably add a second heavier cabled ground on my radiator and a smaller flat steelbraid and solder it on to my heater core nipple. I am assuming this voltage spike occurred when I installed my aluminum heads. It's going to get worse now because when I flush my radiator....and my block...I am going to install my Aluminum Mancini Water Pump and housing.