I have new chrome valve covers on the rebuilt 390, I just replaced the cork valve cover gaskets, drove the car about 50 miles and can see oil seeping from the gasket when I put the car on my lift, I tried to scuff the seat on the valve covers and cleaned the head,
I think the engine rebuilder also used cork on the oil pan gasket, and that also seems to be leaking
Is it better to go with the rubber gaskets or another type
Sometimes people over tighten the bolts and bend or crack the valve covers so make sure the mating surface is flat and not cracked or bent. Good Luck on your Cougar.
The cork gaskets seal much better than the rubber ones. Installation technique is the key to success. The gaskets need to be glued to the valve covers with contact cement. There must be a dab of silicone sealant placed where the valve cover gasket crosses the intake gasket at the front and rear of each head. Everything must be clean and dry, no oil residue of any kind.
Royce, if there is oil on the gasket, can it be cleaned or is better to get anew set. You mentioned contact cement not gasket sealer.
The oil is seeping out at the back lower corner of each gasket
In my experience the best adhesive for cork or paper gaskets is a product called Gasgacinch. It is a milder version of contact cement. Works well on rubber gaskets and is a good thread sealant.
That’s the low point of each gasket. If they are not attached to the valve cover you have a lot of possibilities for leakage.
You ought to be able to remove any oil residue with some solvent.
I use 3M 1357 or 3M Fastbond 10 contact cement in any case. Follow the direction and glue the gaskets 100% to the valve covers if you want to eliminate leaks.
Thank you Royce I will let you know how I make out
Do you also do the same for the oil pan
Normally I use a windage tray with the two pan gaskets glued to the windage tray and then use The Right Stuff to bond the gaskets to the engine and to the pan. You don’t want leaks there.
Had to look up windage tray, Thanks for sharing Royce.
I saw many examples of homemade trays. Do you customize or is there a manufacturer product you like?
Getting all the info on one thread for future use. 
Thanks again.
Don’t over torque the bolts either, you’ll distort the cover/pan flanges - 100% guaranteeing a leak. The factory torque spec should be followed.
The trick which cork is using a spray adhesive like Royce mentioned to hold and seal it in place and while it works and was how it was back in the day I prefer a newer style. Felpro makes a gasket that is much like the stuff used on modern engines. I have used these on oil pans and valve covers with great success and especially with oil pans it makes installation much easier. They are reusable to an extent too. Such as making valve adjustments and having to take the covers on and off. check em out https://drivcat.com/overlays/part-detail.aspx?brandId=FL&pNum=VS%2013264%20T&partType=Engine%20Valve%20Cover%20Gasket%20Set
I use factory Ford ones or the copies sold by Milodon.
While the use of a sealant or adhesive is common practice I installed the cork valve cover gaskets on the 289 dry. This is perhaps a mistake. Yet no leaks thus far.
I used these gaskets on my 289, no leaks
Need to see where I can get these for the 390
Royce, Do you put any of the Contact cement on the head, or just the valve cover and where it contacts the intake.
Do you recomend a cork gasket for the oil pan and also using the contact cement
No, I glue the gasket to the valve cover. I put a small dot of silicone sealant on the place where the intake manifold meets the cylinder head on FE engines. I use cork gaskets on FE oil pans too.
If there is a windage tray I glue gaskets to both sides of the windage tray, then use “The Right Stuff” (reinforced silicone gasket maker) on the engine and oil pan surfaces.
Sometimes I build an engine with no windage tray. In that case I glue the gasket to the oil pan, then use silicone all the way around in a small bead.
The valve covers occasionally have to be removed, that’s why they don’t get as much silicone. They can be re - used many times. Simply replace the silicone each time.
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Royce, Do you put any of the Contact cement on the head, or just the valve cover and where it contacts the intake.
Do you recomend a cork gasket for the oil pan and also using the contact cement
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Thank you Royce, I will try this
I use the same technique as Royce. However, be sure that the intake gaskets are not above the v/c rail cutting your v/c gasket. You sometimes need to trim them back.
Thank you I will check