The thing to do is map out your grounding locations prior to PC then put a bolt and washer at those points.    When you remove the bolts and washers after PC, you'll have a real nice spot for grounding.

I did not do that so I had to clean them up.  An 80 grit sanding drum on a Dremel worked well for me.

Have you checked continuity from a head bolt to an engine mount bolt?  If I remember correctly, Rogue1970 actually put a ground strap from a head bolt to his frame (maybe a head stay mount) to cure his grounding problems.