another overheating problem
Yes, you might get away with .004" or .005" warpage, but not .039". Heck the original head gasket was only .022" thick! So I am not sure where the .039" max warpage came from. Maybe .0039" ?
.015in/.381 is the size I used
The .381 is .381 mm, which is the metric conversion from .015"; the OP appears to have a set of feelers that is marked in both English and metric. So that explains that... LOL OP, you need to ignore the .*** numbers which are in mm, and use the .0yy numbers on your feelers, which are in inches.
You are being guided onto the right path to have the head pressure-checked for cracks. It sounds like this started after the rad blew and things over-heated, and that is a good situation to lead to a warped head. Hopefully it is just warpage. And a straight edge to check for warpage needs to be darned precise. (I checked mine against a precision straight edge use to true up the ways on lathes...)
I have never tried the dye that RR mentioned; that sounds useful. You can also sometimes borrow a gauge that goes in pace of the rad cap; then pressure builds up, you can see the pressure in the cooling system, and more importantly, there will be pulsing in the gauge indication if you have a head gasket leak into the cooling system.
As for the hardened oil 'gunk' around the plugs, if you have the aluminum cups around the spark plugs (called 'drool tubes' LOL), they have gaskets around the upper edges to seal off the oil vapors behind the cups. These gaskets rot and distort and leak. A new set is just a few $$. Also, if I recall right, the compression seal ring on the plug shank should be removed if you have these cups in the head. (With some exceptions on the very earliest models....)
