Vacuum on 1968 GTS 340 4 spd.

Hmmmmm.... 145 to 175 is about 1.5 points in compression ratio! That's gonna take a lot more than a coupla cc's from .010" difference in head gasket thickness, or the 2 cc's chamber difference in the 2 SBM Edelbrock head types. That is more than 10 cc's of difference...which would be greater than .050" of deck difference, or something in the valvetrain on one side versus the other.

Did you record the actual compression numbers? If so, it might help the list them so we might spot another pattern in the numbers.

Was the right bank where the shims were placed? If so, then the lifters on the side with the shims would be pumped up higher. If you then took the shims out, and did the compression test without running the engine first, then the lifters on that side could all be hung open and ruining the compression on that side. Oh, maybe not....sounds like you re-started the engine after removing the shims.

BTW, the .030" shims would have opened up the pushrod side lengths by the rocker ratio times .030" , or about .045".