Should I keep these heads?

Sounds like a 596 or 051 casting smog head. You should cc the chambers. Factory is between 66 and 72.5 cc. Only way to reduce chamber size is a magnum head or cut your heads down.

Smog heads can flow just as good as a 340 head. More work is needed though. They do have hardened valve seats. Air injection ports need to be blocked.