Hi,
I agree with the previous suggestions.... Leaks at the bolts or cap are rare if internal vacuum is correct. You might check your PVC valve is working properly.
Do you have a bypass oil feed sending more lubrication to the valves? If so, you might need to restrict it to reduce the amount of oil flowing up to the top end.
There are hard "fiber" or hard rubber washers that can be used under the nuts, to help prevent any leaks there. Nylon washers might be easier to find and also might do the job.
More likely is leaks from the gasket. As already suggested, first check that the cover is "square" and hasn't been over-tightened (also a common cause of leaks on the sump pan).
The silicone gasket is great. If you want to keep using the original valve cover, one of these might be just the ticket. Keep in mind that there is no sealer necessary with these and you can use one over and over, so consider the price of maybe a half dozen or more standard gaskets for true cost comparison. More info at
https://www.jmwagnersales.com/T6SiliconeGasket.htm
Unfortunately silicone gaskets are only being made to fit the original, stamped steel valve covers. They won't work with the alloy valve covers.
The alloy valve covers seal well, too, thanks to their rigidity. They also help dampen valve noise, and at least theoretically should help the engine shed a bit of heat.
Either type of cover might be helped by installing a new cork gasket, if you don't want to spend the money for the silicone gasket.
Try soaking the cork gasket in warm water for an hour or so, before installing. That helps it conform to the head and cover better. Any moisture is "cooked off" as soon as you start the engine and warm it up.
You can also use something like Hylomar sealer on one side of the gasket. It will seal very well, but it's a bugger to get apart and completely off, when the time comes.
Perhaps use RTV sealer on the other side, or both sides, of a cork gasket to make disassembly easier and possibly be able to reuse the gasket once or twice.
If you use a sealer, be very sparing with it. Just a thin coat is all that's needed. You especially don't want it forming a ridge on the inside, eventually flaking off and possibly working it's way into the oil passages and such.
Hope this helps.
Cheers!
Alan Myers
San Jose, Calif.
'62 TR4 CT17602L