Sorry to sound so dumb (I've worked on LOTS of dirt bikes....engine swap on a truck..first time) I've read over and over that the 4.0 will leak if not done correctly. I read on THIS forum to use cork for the valve covers and I did. They said to silicone the bottom side (above the exh. manifolds) to stop oil from leaking. Dumb I know....but silicone BOTH sides of the cork? Or just the top of the head? Also, ...oil pan, do I need to silicone all surfaces along with the rubber gasket? ANY other tips as to where TO silicone and NOT to would be appreciated.

 

Cork is better long term because it will swell from oil touching its inside edges, so self tightening, rubber gaskets are used when you will be removing the valve covers frequently because they can be reused
i.e. racers will use rubber gaskets on many parts, because they will taking things apart and don't want to buy new cork gaskets every time

But if you are building it to last, cork is the way to go

Up to you really, RTV(silicon) helps hold cork gaskets in place and will seal any imperfections in metal surfaces, don't use alot, or when you tighten things up the RTV will squeeze out inside the engine and it makes little silicon ***** that can come loose and clog up your oil pick up screen
So yes both sides of valve cover or oil pan gaskets is fine, its not a "new" engine so will have imperfections, lol

If possible flatten where the bolt holes are in the valve covers and oil pans
These will be deformed from previous use, i.e. over tightened which bends the thin metal, so if its not flat then when you tighten up the bolts it won't put as much pressure in between the bolts so you can get leaks there
This is why "spreaders" are used on valve covers, because the thin metal is deformed so can put enough pressure in between the bolts

And yes the lower edge of valve cover is where oil pools when engine is running and after its shut off, so thats where oil can/will leak out

Inside of oil pan gets oil sprayed/thrown at it from the connecting rods spinning, so can leak

You also always put on a new rear main seal, on the 4.0l its often recommended to use a "sleeved" rear main seal, as the crank surface the seal rides on is often not as smooth as it should be

 

Thank you sir....owned a couple Rangers....first go around installing an engine.