What about doubling up on your gaskets?

 

What i have done before is buy 2 sets of 3bolt flanges and weld them together making the flange like 1/2 thick. Then i just put some high temp rtv on both surfaces and bolt it together with a impact. No leaks. The problem with 3bolt flanges is they are too thin which makes them warp between the bolt holes. Doubling up the flange prevents warpage. I have also used vbands with no leaks either.

Vbands on collectors





Doubled up 3 bolt flange. Welded inside and out