@jayco - I don't think there were the sheer number of 454's in Class C's that there were 460's. First set of manifolds I ever brought to a shop to be re-surfaced after warping and starting noisy leaks, was from a 318 small block MoPar engine. Machinist asked "These out of a Van?" and I told him they were. He said the van doghouse is so limiting on air flow around the engine, that vans tend to be the ones that warp manifolds. Now, take all those far larger 460's, stick them in van "noses" and load them to max GVWR, maybe towing something besides. Recipe for manifold failure. I've heard of it on 454's too, but I think there might just be that many less of them.
@john&gloria - Ryan has emailed me, and he's making slow progress. Found a drill press he could use to accurately make a drilling template on the idea of those commercial ones. Last report, the LH bits he bought would NOT cut the steel in those bolts so he used another brand of RH bits. I believe he got through to the blind end of both broken bolts.
For an OP wanting to re-surface an exhaust manifold: Most machine shop "milling machines" are for heads, and most exhaust manifolds have the pipe connection where it interferes with the plane the milling machine needs to reach the sealing surface. The shops that have been able to do manifolds for me, have a "belt sander." It's a precision machine shop tool, not what we use for woodworking and furniture refinishing. It's called "sander" but it's capable of producing an accurately flat and true manifold gasket surface.
Best I say again, I believe we can reduce manifold problems, by letting the engine stabilize at reduced speed/load, or idle, before shutting down after a hard run.
Also, Engine Cooling System has to be 100-pct including Fan Clutch and Radiator Core inside and out.
And, no exhaust system restrictions.
