340 deck

If it’s the coolant passages, that was not an uncommon practice years ago especially for race prepped blocks. If you were to look at factory original head gaskets compared to replacements some of the coolant passages in most are partially or fully blocked off. This is or was done to change the coolant flow thru the heads and if I remember correct helped balance coolant flow. But that’s been a long time so take it with a grain of salt.