Do not dismiss the notion that your valve covers/gaskets are still leaking just a bit, especially the passenger side back. Rip up a nice clean white bed sheet and tie it completely around the head and valve cover so that it's snug. Take the car for a nice hard run, let the engine cool off, and remove the sheet. If there's even the slightest
oil stain then you're leaking or "misting"
oil. When that happens it almost always trails down the back of the block and block plate and then drips off the bottom of the bell housing or from behind the starter motor. Now, FWIW, here is what I did to finally stop every last lick of valve cover leakage. No guaranties that it will work for you, but if you fail the bed sheet test you will never, ever, not have a periodic drip on your floor. Even one drop of
oil on the floor bugs me.
These are the steps I used, and it took me over 10 years to finally achieve success.
Remember, I have solid, flat tappets so I have to be able to periodically remove my valve covers to check the lash. These steps assume your valve covers are reasonably true and that you are using studs with your heads and covers. It also assumes that the spots where your intake manifold and heads meet up are not "too bad."
1) Clean up the rim around the heads where the gasket sits thoroughly. I like to use 3M General Purpose Adhesive Cleaner 08984. Get it clean enough to eat of off and then put a small glob of Open Gear Grease on the four places where the intake manifold meets the two heads, and in to the little crack where they actually meet up, and then put a light smear of it around the rim of the head where the gasket sits.
2) I use the Fel-Pro SFL1632 Blue Stripe gaskets. Place the gaskets down on the head, over your studs, and fit them down lightly with your fingers.
3) Clean the rims of the valve covers with 3M General Purpose Adhesive Cleaner 08984 and put a light coating of 3M Yellow Super Weather-strip and Gasket Adhesive 08001 on both the top of the gaskets that are on the heads and to the rims of the valve covers. DO NOT PLACE THE VALVE COVERS ON THE HEADS UNTIL YOU HAVE ALLOWED THE CEMENT TO "FLASH DRY."
4) Allow the cement to flash dry for about three minutes and then place the valve covers down on the gaskets, but do not bolt them down. Tap them down lightly with a rubber mallet and let them dry for a couple of days.
5) After a couple of days, torque the valve cover bolts down to sixty inch/lbs. (five ft./lbs.) in increments of two pounds each going around the valve cover just like you would do a cylinder head. I prefer to use my inch/lb torque wrench that has a pointer instead of a clickable torque wrench when you're down at low inch/lbs numbers. DO NOT OVER-TORQUE.
6) Take the car out for a nice long run so that everything gets good and hot. After the engine has thoroughly cooled, re-torque the bolts to five ft./lbs again. You will be surprised at how loose they have become after the first run. Do not exceed five ft./lbs. in torque and you should not have to touch them again.
Hope this helps.