I use ordinary 3-hole header gaskets with excellent success on my fabricated exhaust systems.  Unfortunately your flanges are not standard header size.  Perhaps the material is available in sheets so you can cut your own.  McMaster-Carr may be a source?

Silicone sealant works fine so long as it is close to a water jacket, like pipe to head flange, or down around the collector flange or further back

Regards
pat

Like Pat, I often use one of the high temp RTV's on the flange to head surface (THIN film) but the last several headers have had 1/2 inch flanges instead of the usual 5/16 inch material and, as surprising as it sounds and, with proper matching, the header to head pipe connection has needed NO gasket at all.  The half inch plate is not very heavy once it is fabbed into a standard three bolt shape.  I just used a standard flange as a template. Oh yes, I also went  to 3/8 inch SS bolts and nuts on the flange plates.
I have heard, from one of the local drag racer types, that on SBC's  the thicker plate results in the header assm  not loosening as often as thinner plates---seems logical.

Point of fact---I generally use a slip fit on any exhaust systems on the vintage road race cars.  They sit to low for clearance of a big flange and rules ban floorpan mods.

Rod

Innovator....

try Welltite. It is a cousin to Hylomar and was discovered/invented to seal WW2 tank head gaskets. I have not used it personally but it gets rave reviews in the UK.

BTW...you dont have Mcmaster-Carr over your way.

As a suggestion if you do another exhaust system, make the flanges a little thicker. Use .062" compressed asbesos sheet and cut your own gaskets. If compressed asbestos sheet is no longer availiable over your way, use other high temp material of at least 600 F.

Also instead of flat flanges you can use Marmon clamps...rock on over to Demon Tweeks and see what they have....a lot cleaner but more $$$ (lbs.)

good luck