If you remove the oil pump on a big block or tear it apart to inspect it, you need to replace the gasket and seals. If you go to a parts store, they'll give you a large O-ring that is used to seal the plate to the pump body. Chrysler no longer carries them (there is one dealer in the US that has one, and it's not for sale accoridng to the computer)

The gasket and seals are still available from some sources.

Melling has them with part number M-63016. O'Reilly Auto Parts and Jegs has them available.

They are also available from Macnini Racing. 440 Source sells just the paper gasket.