On the back of the valve cover gasket is a half moon shaped rubber insert that can leak. If the large leak you are describing is at the passenger side rearmost part of the block where it mates to the head it is almost for sure the head gasket. There is a sealing ring/area in the head gasket at an oil passage there that fails frequently.

If you can, replace the head gasket before oil gets into the cooling system.

If you can do a compression test before pulling the head you may save the time of doing a valve job if you are on a budget. May just need seals and resurface. If any guides are worn though it is time for a valve job.

Good luck!!!

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