Randy, other than the 68+ sideoiler service blocks the only late model 427s I know of are the marine blocks. Don't know if they would have the
oil pressure relief valve like the 390hp, 406 and 63-65 427s. what year is your block? Is it a marine block? Also I don't think any 27s had this after 65 when the sideoiler came out. But i'm by far no expert. Glenn.