Update. Since original post, I bought a full gasket set on e-bay for $35. It must have predated the 318 since the label only states 273 cu in (I'll post a photo later). I thought I scored big until I measured the head gasket bores as the usual over 4"D. Perhaps 3.625"D gaskets never existed. Has anyone ever pulled an original head? Did Chrysler already design for the later 360 bore in 1965? The compression loss is very minor (see calc above). I was more concerned with issues from the thin gap (quench, carbon chunks forming, etc), but apparently no problem.
