So, My heads are corroded to a point (one cyl. 1) that the normal grey merc / CMI gasket will not cover. I need to make a custom gasket, wider then the standard to seal. I don't have the time to have the head fixed at this time. To bridge the gap but the standard gasket is not wide enough. Where do I get a sheet of high temp exhaust manifold gasket, the grey type? CMO header flange is wide enough to get around the corrosion, just need a wider gasket for now. At least 3" x 24' +-
THX
P

we run solid copper ones but if its a iron exaust manifold we never run a gasket just a clean surface and a film of the right stuff by permatex it never fails!

Fel Pro and MrGasket both have the material available in sheets. I think Fel Pro might even have a "cut to size" gasket that is bigger than needed that you can trim.

Mr Gasket 5961 if you can find somebody that has it. Looks like Summit has their own brand available:
https://www.summitracing.com/parts/mrg-5961

In your situation Permatex HI Heat Copper or Red is the solution. It'll also stop the head from getting worse. Smear it on one side and put the manifold in place. The do the other side. I haven't used exhaust gaskets on anything in the last 25yrs after an old timer told me it works.

Have you looked at Remflex gaskets. Much thicker and compresses. They won’t burn either. Fixed a lot of poor fitting exhaust problems