Hi,
I know there are heavy duty (thicker) gaskets available for the 4-cyl. Triumphs, would have to assume that someone (Moss Motors, Victoria British, The Roadster Factory, British Parts Northwest, British Frame and Engine, Goodparts, etc., etc., etc.) sells one for the 6-cyl. cars, too. Keep looking.
However, a thicker gasket generally isn't necessary.
First, you mention a header and that might be part of the problem. Often headers need some "fine tuning" for a good fit. This is because they are essentially handmade, plus welding can cause some warpage as it cools. Often some milling of the headers manifold face is needed, to make it as flat as possible or to adjust a little for a particular car. Some headers just fit better than others, sort of depending upon the manufacturer. And, there can be other factors. One header I installed on a TR4 couldn't be tightened at all because it had a thinner flange than the stock exhaust manifold and lacked spacers at most of the hold-down points, to make it flush with the intake manifold. Once some spacers were welded onto it, it could be pulled up nice and tight. You might want to check with a local shop that does header installations to see if any of this sort of work is needed on your car.
You might be able to spot some of these problems yourself, if one or more of the fasteners seem to tighten up first, while there appear to still be gaps. If you are using a torque wrench to tighten, it might also be as simple as a bunged up thread on a mounting stud or a bad nut that appears to tighten, but really isn't.
Unlikely, but possible, is that there is some fault in the face of the head itself, at the exhaust ports, causing a small leak that eventually turns into a big leak. Worth checking carefully, but I doubt you'll find anything. The mating faces of the head and the manifold each need to be nice and clean, too.
One problem with some heads happens if they are milled to increase compression. This is usually only when a lot of milling is done, that repositions the gasket and manifolds so that some obstruction causes some interference. For example, I know the top edge of the block of the 4-cyl. TR engine needs to be chamfered a bit, to increase clearance, if the head is milled more than .120-.150" or so. If that's not done, the block will usually interfere with the manifold gaskets. I don't know whether or not this might happen with the 6-cyl. head, but it's easy to check and correct if need be.
Close inspection of all these areas, threads, etc. might find the problem. Use a straight edge on the face of the header and the head too look for interferences and/or if either is warped in any way.
Also, if the exhaust system puts a lot of strain on the header, that might be part of the problem. A clue to this is if you have to really lever or force the parts together during assembly because they don't align well, or even if engine torque strains the system while driving (a flex joint installed close behind the header might solve this latter problem). The continual stresses can eventually cause leaks, or even worse, can crack mounting studs or the manifolds themselves.
Permatex, for one, makes a copper gasket sealant that's available in spray, brush-on cans, and squeeze tubes. In this sort of application, spray on is likely the best to get a nice, even coating. Most any local auto parts store will have this sealer.
Hope this helps. Keep us posted!
/ubbthreads/images/graemlins/cheers.gif
Hey there Guest!
Hey - did you know if you click on the title of a thread it will take you to the first unread post since you last visited that thread?
but were afraid to ask: 
STOP!! Never post your email address in open forums. Bots can "harvest" your email! If you must share your email use a Private Message or use the 
smilie in place of the real @
Pretty Please - add it to our Events forum(s) and add to the calendar! >> 
A friendly reminder - be careful what links you click on here. If a link is posted by someone you don't know, or the URL looks fishy, DON'T CLICK. Spammers sometimes post links that lead to sites that can infect your computer, so be mindful what you click.