A common problem isn't the limited gasket surface, but the fact that there is almost no vertical spacing between upper and lower bolts.
The extreme length of the primaries means that huge leverage is exerted against the flange when the collector moves even slightly.
Anything you can do to tie the bottom of the header to the engine (not the chassis, or the exhaust to the muffler) will help but it must be very stiff left to right. The easiest point of attachment is the collector bolt pattern. Make 2 1/8" or 3/16" thick tabs with a hole for a collector bolt and install them at the collector ring, and use these as the take-off for the struts. Weld or bolt tube to each tab, thin wall is fine, 3/4" minimum (like conduit), or an "L" - don't use strap, it's too flexible. Run 1 from the collector ring across to the block wherever you can find a bolt or stud, and the other to either front or rear (it will be on an angle), both is better. Can be to an accessory bracket as long as it's strong like power steering or AC.
I've had good luck with wire cable, but positioning it is more critical, it can't make bends, and it has to be tensioned so it's dangerous.
