A BETTER WAY TO INSTALL HEADER GASKETS
The biggest reason for leaking headers is incorrectly fitting header gaskets. If you will hold the typical asbestos gasket up to the headers, or the head, and align the bolt holes, it will illustrate the very poor fit and alignment. We have not tried the newer, more expensive copper or woven gaskets, because the following technique works so well for us. However, if you prefer to throw money at the engine, rather then taking the time to make a good seal, this procedure is probably not for you.
If headers are used, here is a better way to seal them: Either cut standard asbestos header gaskets into three sections, or make the three sections from "Mr. Gasket" asbestos gasket material. The Mr. Gasket asbestos gasket material is available in two different size sheets, and either will work. Making your own individual gaskets assures that the gasket will fit the port while having maximum amount of gasket material for the header to fit against. We cut three pieces for each head large enough to cover the entire flat area around the port openings. A pointed tool, such as a pecking hammer, works well to tap the bolt holes, and also the port openings. By keeping the bolt holes small initially, the two bolts will hold the gasket accurately while cutting the openings and outer edges. Also, file/cut the outer edges of the gaskets to fit to the edge of the raised flat port areas on the heads. After the openings are cut by tapping around the edges, and before installing, open the bolt holes to a slip fit so as not to interfere with the installation. By cutting the pre-made gaskets into three parts, it is easier to center each over the ports, thus providing a better chance for the header to actually have gasket material between it and the head! Assure that the openings can be centered with the bolts installed. Glue the gaskets to the heads using a thick layer of high temperature silicone seal (RTV) (high temp black looks best). After the sealer is set up enough to prevent movement, wipe a generous layer of the RTV on the outside of the gasket and let dry completely. Trim either the commercial or fabricated gaskets to fit the port outlines on the outside with a coarse file or rasp, Now the gaskets are in place and not a problem when installing the headers. There is no ugly section between the ports to discolor or get in the way, and best of all, the extra layers of cured high temp sealer provide a much better seal. After several hot/cold cycles of engine operation, firmly retighten all header bolts. Again, if headers are used, do the same to the collector gasket. Glue it in place on the header using the high temp RTV, and after it is setup, wipe a good coating of sealant on and let dry. Before installing the pipes, insert a layer of wax or plastic paper roughly in the shape of a gasket over the sealant, and the pipes will then disconnect without tearing or affecting the collector gasket. I have used the same set of collector gaskets for an entire season even though I opened the headers each time we went to the track (thankfully, we finally realized we could run about as quick/fast with a full exhaust system, so no longer have to disconnect the pipes). This technique is particularly appropriate for the 7K3 heads, because it is not necessary to tighten the bolts/studs as tight in order to obtain a good seal. It will also work great on cast manifolds, except that the center divider in 4-6 and 3-5 gaskets may not be required. It will not replace the work of cutting and reshaping headers to correctly fit over the port openings, but it will assure that any headers will seal better.