Softened water is water that has been run thru a water softener, removing most of the dissolved minerals. Distilled water is boiled water vapor, recondensed from steam into water, which has no minerals in it. I have been running distilled water and anti freeze in all my vehicles for years - no issues. There are no minerals to collect on the radiators internal tubes to clog them.

Softened water is any water that is un-distilled and then has salt (Sodium Chloride) and potassium added. Good for your laundry, not good for drinking.

Distilled water makes good coffee. But it should not be drank as a main water source due to the lack of minerals.

Tap water all depends on where its from and how its treated if at all. Well water may be full of good and bad.... Tap water from a Miniciple source can also be full of good and bad....

I have for the last 35+ years just added a 50/50 mix of distilled water and good ol GREEN stuff.

But with this 410 Stroker build I am installing an anode into the front and rear block drain plugs. There is enough room for a 0.90 inch long Zinc or Magnesium bar. I bought 5 of these for $20 and am just cutting them down to fit. The end I cut off can be threaded and used later so I really have 10 on hand. I will replace them every 2 years or so. See picture below

https://www.ebay.com/itm/Martyr-CME0-1-3-4-L-5-16-18-Thread-1-4-3-8-Dia-Plug-Engine-Pencil-Zinc-Anode/254316970289?hash=item3b36790131:g:~YAAAOSwZGFdQqeh

EDITED> The Ebay link for 5 pieces of these Anodes
https://www.ebay.com/itm/ZINC-ANODE-PENCIL-WITH-PLUG-70-E00C-SIZE-1-4-X2-PLUG-1-8-NPT-5-PACK-MARINE-BOAT/184015710240?hash=item2ad830e820:g:O6sAAOSwF1dUPrmk

I am not sure how much current is or how long any current is actually running thru an Alum Head. as AndyF mentioned its effectively a bus bar, and I can't believe any current induced into the head connection "likes" the coolant instead of the massive amount of Alum to flow thru. I believe electrolysis occurs and is a concern rather independent of any typical grounding scheme, and I have yet to read anything that makes for a case otherwise.

At FCA we have an EMC lab. They bombard new cars with radio waves and all types of electrical interference. All the new heads are aluminum, many aluminum blocks as well. But no corrosion issues. Working in the electrical lab I tell tell you that grounding is taken very seriously. I have yet to see grounded heads or a full ground cable from a rear mounted battery.????
Doug

OK, going from the factory location up front I will ground using a 2 gauge cable to the front of the left cylinder head. But I also run a 10 gauge to the core support. Then from the backside of the right head I run a 6 gauge strap to the firewall at the voltage regulator. I also like to run a ground strap from the alternator to the block.

When running a rear battery I ground using a 1 gauge wire from the battery to the starter's lug on the block. I also run a 10 gauge to the rear frame rail and a third long 10 gauge up front to the core support. Same 6 gauge from right head to firewall.

With 45 years as a Telephone repair tech, Bonding and Grounding is VERY important. All connections should be clean. Then the nuts torqued to 40 Inch Pounds.

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.

using a electrical meter to measure conduction you need to use the MHO scales, not the OHMS scale
I have ben advise to use those measurements in very low voltage circuits, not that I did it though

I don't doubt that observation one bit. However the reasoning behind it, vs what we are discussing, is I believe the modern OEM's to save wire costs, complexity, and by more effective design and attention to detail, have been able to produce a chassis/unibody that forms a nearly universal ground plane for everything electrical on the car. Our decades old cars did not have that consideration, and also suffer from a decent amount of weather induced corrosion, etc. I see an added ground directly from battery to engine as a huge preventative measure of future electrical gremlins in our old cars as we add more electrical components..

Exactly.