With all of those bolts or studs clamping the heads down, how could there be any difference as to which is grounded?? When using an ohm meter on block and heads, I get zero ohms. Type of gasket material would make zero difference. It even shows a 0 ohms reading from block to oil pan sealed with a thick paper gasket and silicone.
I've always ran a ground from frame to starter mount, and from frame to battery in trunk, Zero corrosion issues on the alum. heads.
I think a better test than Ohms would be testing for Voltage Drop at each point. I'll bet there is some but very minor.
Ohm drops (gains) would be in the 0.00010 Ohms and most digital 'Auto Ranging' VOM's will not see that.
