Have you ever used the following header gasket?
https://www.summitracing.com/parts/fel-1467
This is the one I have for my 306 build Ive not had any problems out of them at work when swapping out blown out header gaskets of other brands.
The other thing is you have to make sure you have your header bolts as tight as you can get them to prevent blow out as well. I like to start the first and last bolt to hold the header, drop the header gasket in start the other bolts then I start in the middle and work my way outwards and I go over each 3/8" header bolt with a combination wrench using the closed end getting the bolts as tight as they will possibly go.
Now we did have one at work that wouldn't seal, but that ford 351W engine had some Chinese heads on there and the headers the guy bought was Chinese made as well to save money and nothing fit right. In those rare cases you only have a copper header gasket to try and stop a leak at the head.
