1 Broke Dad,
If the leak is indeed between the case and the block, you're in for a big project. You can try to clean it and stop it with sealer from the outside, but that is really only temporary. This is apparenlty a somewhat common problem with 12-valves.
You may even be able to stop it for a while by removing the cover and tightening the case bolts. But when the time comes to address it properly, plan on shooting a weekend. It's a significant bit of surgery.
To replace the gasket behind the case means you have to remove it. That means the p-pump, vacuum/PS pump, front bumper, radiator brace, radiator and intercooler, damper, fan, case cover, and finally the cam has to be removed. To pull the cam the rockers are removed and the tappets pulled up and held in place with wooden dowels. Then you remove the case and replace the gasket. Now put it all back together!
When I did mine I chose to put in a Piers cam just for fun. And, of course, kill the KDP if not done already.
J.