I changed the manifolds on my 258 a month ago and they are leaking now! Not sure if they have been leaking the whole time. Anyway, this weekend I am planning on reinstalling gasets...any tips. I am so tired of smelling like I have been huffing on the exhaust pipe, when half the time I really haven't been, Anyway, I want to get this finished to do more fun stuff...like drive the thing. ANy tips would be appreciated.
258 Manifolds have a history of warping. You may want to take the mandifold to a machine shop an have it machined so it's surfaces are true to the head. I put the gaskets on with Permatex Gold or Orange sealant. It's high temp and will make up for some minor warpage. Torqe the manifold down using the proper torque techniuqe. It takes me about an hour and a half to go throught this procedure. I divide the total torque setting into several foot pound steps. Then after the job's done... I drive it about a week (200 - 300 Miles) and then re-torque the manifolds. After that I don't have a problem.
If you use a straight edge, make darn sure it's straight. Lay it out and test the entire faces on the runners. It's not as accurate, as the machinist will probably mill it to spec. One sure sign it's warped is that the runners don't fit when you try to put it back on the head. If that happens, you'll either have to buy and use a runner spreader or elongate mounting holes.
You're right about the intake not warping like the exhaust manifold does. I'd would replace both the gaskets at the same time, exspecially since often their fastened together. It's a case of in for a dime... in for a dollar. IMHO removeng the intake manifold does make removal and re-installation of the exhaust manifold much easier.