I always had good luck with those steel/fibre gaskets that come in the gasket sets. Just make sure the bolts stay tight. 

 

copper header gasket and keep bolts tight.

I bought a set of SBC block-huggers off a friend (along with an intake, carb, valve covers and air breather). He could NOT get the header gaskets to seal well at all (and I knew this when I bought them). He had tried the copper gaskets too and they also leaked. I too had issues with them not sealing (no surprise there) and I even cut the flange between the tubes so each pipe would have a better chance of conforming to the head ..... nope, didn't solve the issue so I welded the flange back together (after bolting them to a junk head). Down the road, I finally did get them to seal well (the cutting and re-welding didn't do it), by simply buying a regular (not copper or anything else special) gasket that correctly fit the size of the port on the head and the tube of the header ...... no more problem, I didn't soak them overnight, I didn't coat them with anything, I just found the correct gasket for my application and my leaking/burning issues was history. As I found out, there is more to buying header gaskets than just walking up to the counter and saying "I need a set of header gaskets for a SBC".

No idea whether this approach will solve your issues but it's how I solved my header to head leaks 

 

I wont use header gaskets any longer. I use High temp silicone on all the flanges. I put a 3/8 bead of silicone around all of the flanges and let it cure for a day. Install the headers and no more problems. I have installed many sets this way and have never had a failure. I thought everyone new this trick.

several fixes that wrk

first has been mentioned - using the factory style steel sandwiche gaskets - but I also add the copper high temp RTV to them

second is a little more work but always works and never ever leaks

take regular household tinfoil - run off about 2' and fold it as many times as it takes to make a 1" wide panel by the 2' long length of the foil - then fold the layered 1" width in half - now take the tin foil and insert the folded foil over the inside edge of any gasket you are using - so it now covers both sides of the gasket, and slowly fold the tinfoil around the opening - so when you are finished you have approximately 1/2" of tinfoil lapped to both sides of the gasket opening - continue to fold the foil so that the end over laps your starting point by a good 1/2" and cut the remaining length of foil off
Now - when you bolt this down the block and the header will sandwich the layers of foil causing it to fill any voids, and make a very good seal

The layers prevent burn through and you will never have another leak - this also works on collector gaskets too

Double felpros. end of trouble. If you look at most headers they are not machined flat. In many cases there is simply not enough crush to absorb the irregluarities. also some heads have a small step the interferes. Cheap head gaskets are a pain. Double felpros which will hurt your wallet will probably still be there when you retire the car 20 years or so from now. BB mopar which is similar header style to SBChev often blow the upper centre . When running a car competitively we could not afford sudden changes in ET. Blowing the section usually cost 1.5 ET instantly plus some MPH. After fooling with all the magic solutions I tried double felpros. Last header gasket repair I ever did on it. Now I use them as a matter of course and once I have shared this info have zero sympathy for those who don't.
don
re sudden et changes We ran once at Luskville 5 consectutive 11.560s. (Same weekend I spanked Arnie's 454SS with the V10.)

Sometime the headers are just crap. I had one set on my dart that could never seal, even after cutting the flange and having it machined flat. I gave up and replaced it and the new set never leaked. Too bad they hung down too low and found a raised manhole cover....