with 4&6 reading so low I would hope for a head gasket or a cracked head on that side, maybe even some head warping from excessive heat. I had seen it happen a bit out here where it gets so hot. occasionally improper torquing of the head bolts could lead to something like that, though this looks worse that any case of that happening that I've seen before. another wet compression test with fairly consistent results may help pin it down.

62' GMC Fleetside 3/4 ton 235 4-speed.
Time makes fools of us all.