If I may, I would like to throw at you some logic and something you can do to confirm what you might want to do with that head.

The posters are correct in saying you do not want to leave the sealing of the head to the block entirely on the head gasket. It is put in there only to "fill in" slight imperfections of machining. As I have said nothing is perfect in this world.

A cutter tool no matter what type (even grinding or lapping) creates channels and when your are dealing with heat, gases from combustion, petroleum and some possible corrosive liquids they are darn small. We are talking about possible, no for sure, molecule migrations.

I have worked where we pulled molecules of air through stainless steel bar stock under a high vacuum. Ended up using sheet steel because of its "compressed grain structure" to minimize it.
Only a Part of the theory in using metal rings around the cylinders of head gasket. Head gaskets have changed a bunch.
You might want to check with gasket manufacturers to see how much they can tolerate as they crush and perform differently today! There is cold forming and interlocking layer technology.

By the way a perfect vacuum does not exist because of radiated energy (still a particle) kept passing through our cascaded helium refrigerated space!
An example later found was, what we call a Neutrino today! We surmise that they can pass right through the earth! We are still looking for those "kind of things" up in the mines of the Dakotas.

Those warpage limits mentioned in manuals, were actually written in by some "college boys" who were taking notes while standing around old time mechanics of the past!

Imagine the Sargent of the flight/motor pool in T.V. Show Blah Blah Black Sheep with Robert Conrad. In his mind collage boys had other interests than ever getting their hands greasy!
The reason he munched his cigar! (:-/

The reason I know this is because our Machinist Handbook, a reference guide, started back when apprenticeships required some "standards" along with reading and writing! Feeds and speeds, you name it!

The .004 to .008 reference is a "rule of thumb" number "mumbled" out there. It's an average number of what should be used on heads in general or in reference for mixtures of aluminum in modern heads of different widths from various engines.
If you take that .004 across a width of head and divide that length into the overall length you end up with "approximately" .020 from the extreme lengths of corner to corner.

With manuals you have to watch out for that lazy college boy who plagiarizes! The number you came up with very well could have been snatched from a book on a twelve cylinder in-line motor from England.
I "heard" in my past, that Jaguar or Rolls Royce tried to build engines without any gaskets too! They tried to grind and lap everything. Might explain some of their thinking, less expense and their attempted reputations of the past. Good and ugly, as they may have been!

My suggestion would be to bolt the head back down on the block with the valve buckets removed. Put the cam back in place. This so you can later turn it free of spring pressures. Put the complete assembly back on the block.

Try bolting the head down on top of an oil soaked paper towel of a continuous solid sheet. Then see if you can turn the camshaft by hand only!
When you remove the head you should see that all of the oil was squeezed out evenly and pretty much complete flat pattern.
A plastic film might give up a mashed looked too!

You want a cam that turns easily and a head that's mashing flat into that plastic/paper fiber test bed.

I have never done anything like this but it is a method to play around with. Nothing to lose but time and some brain energy.

I have made refrigeration gaskets. Once for a large strainer type oil trap/accumulator housing. The capping plate was defective. I machined out the defect and then made a gasket out of a thin burgundy colored, hard faced paper, of a desk file folder! I used it for the crossed overlay of fiber pattern I could see in its surface.

I was desperate to get a roof top, three compressor staged system back online until I could get a replacement. Surprisingly, it never leaked and I was able to leave it in there for a very long time. Actually, until I had to replace the later on severely corroded condenser coils.

It got my employer by that made them very happy. It made it a good experiment until funding became available.

Have at this test idea, it's your mission if you choose to accept.....the experience!
Phil

The .004 mentioned in Haynes is the max amount the head can be distorted and installed as is. The head bolt torque is high enough that tightening it will straighten it and allow it to crush the gasket and seal. Anything more than .004 and the head has to be straightened and surfaced before installed.
The problem is if you actually have a .020 warp lengthwise. That's a lot, and the head should be straightened before surfacing. The cam bearing bores will not be in alignment if the shop just machines a whack off the head. A good shop can preload the head in a jig and then heat it to get it close to flat, cool, then machine a bit off to get a good surface.
You also have to consider that that amount of overheat the pistons have probably wiped out what little skirt they have and the rings may have lost tension. Even the blocks can distort and the main bearing bores go out of alignment.
So fix it as cheap as you can and run her and see if the bottom end is ok. Maybe it's still good.

typically 5'000s requires re surfacing and 20,000's is pushing the limit for warpage and still be able to use the head.

that said there is good shop near me that can press a badly warped head under considerable pressure and heat for 8 hours or so and then be able to resurface a head for use.

if the shop says the head is too warped to use you might look for a shop that can as above.

i always resurface a head if a HG is to be done.

I only know that the guy who does my machine work once told me that "anything more than .004 is a lot to ask a head gasket to deal with".

When you take into account the effort, expense and down time involved in replacing a head gasket I am not one return marginal components to service; I don't enjoy the work enough to look forward to perhaps doing it again anytime soon.

Good luck,
Randy