Weren't the extra guages in the 68 Shelby's Stewart Warner? As for extrude honing, it does a very good job and I would think that it could help a bit on exhaust manifolds but I wouldn't expect too much becuase the design and layout of a cast iron manifold is as much of the problem as the restrictive size. If you are trying to squeeze a few more ponies out of the Shelby then it might be a good mod but you will need to find someone who has done it to get a real world assessment to determine if it is cost effective.
In looking at the flow of my exhaust manifolds, I am thinking that extrude honing would have minimal effect. Ford exhaust pinches up pretty tight where the bolt holes are, and I don't see a way to improve that.
Too bad, manifolds are a lot less hassle than headers.