Well, this changes everything. So basically, you bought an NOS Ford HG set for your engine? If so, I would feel decent about using that gasket. I still can't help with the squash thickness though. I'd be curious to see the actual HG though. I wonder if a Felpro MLS gasket would still be a better option than OE from 1984.
I think that it could be just about anything matching that NSN number...
If it is a steel shim head gasket (which I doubt it is) then it COULD be a NOS Ford item in which case the thickness is around 0.025" compressed.
If it is NOT a steel shim gasket then it's probably around 0.050" compressed. Typically, when rebuilding a small six with a modern head gasket the head is surfaced in the amount of the difference between published compressed thickness and 0.025" to maintain compression ratio (combustion chamber volume) and distance between lifter and rocker (pushrod length).
the stock ford composite gasket is a decent one to use. and while i believe the felpro brand is better, the gaket you bought is good to go, just install it properly. also the 250 started getting the composite gasket, i dont believe you will lose compression. one bit of advice here, hop on over to vintageinlines.com and grab a head stud kit for your 250
this will give better and more consistent clamping force on the head gasket allowing ti to last longer. be aware that the stud that goes in over the water pump will need closer attention to prevent the stud from interfering with the operation of the water pump, so keep that in mind. by the way you need to check even if you use the stock head bolts.