Source for 273 Head Gaskets??

You guys overthink everything. Buy a set of good Fel-Pro's and move on. I run them on all my 273's.
Fel-Pro 1008 Fel-Pro Performance Head Gaskets | Summit Racing

Or try a Magnum 318 with a 3.906 Bore. Not sure of the thickness. Might have to get a set to investigate.
Fel-Pro 26179PT Fel-Pro Performance Head Gaskets | Summit Racing