I'm assuming you are talking about a big block with the 3/8 exhaust bolt reference. I found these on Summit's website
.http://www.summitracing.com/parts/CGT-C5902-030/
They are MLS exhaust gaskets and that's what Cummins uses on their 5.9 and 6.7 diesels. They seem to hold up very well on those applications, even to the point of the manifolds cracking and falling apart, yet the flanges usually continue to seal up unless the bolts are broken.
I'd give them a try. In fact, I think I'm going to on my 440 with headers. Cheap gaskets included with the headers keep leaking worse and worse.