I use Sandersons copper gaskets with a 3/8" bead of silicone on the header side. Typically, I would use silicone only, but I wanted to isolate any contact from the steel header to the aluminum head and still use the silicone.
On the header, you can run a 3/8" bead of high temperature RTV Black around each header port. You will want to install the headers when the silicone
skims over, usually about 45min to 1hr. Torque them at 25-30 ft lbs. I used a couple of all-thread bolts to help slide the header in place. I realize in some applications this can be very difficult, but it is well worth the effort of not having to retighten header bolts all the time
In a typical application, the bolts will stretch if you over tighten them and become loose because they have no room for movement. Silicone is more pliable than a regular gasket therefore giving more leeway for expansion. This will help prevent them from stretching any further as the header expands & contacts from the extreme hot and cold.
To answer you next question, yes the silicone
will withstand high heat and exhaust pressure; in addition, it comes off very easy when you yard the headers out. A lot of guys I know, and some running as high as 14.5:1 compression, use silicone on their headers without fail!