They need to make a shim like the 'head saver' shims for OHC 4 cyls. Used when you machine the head surface so far it messes with the valve timing. One the right thickness to drop the compression a point on a V-8 would be cool.
They are cometic head gaskets, but that thick of a gasket can cause other issues.
No way will 670s run pump gas on a 455 for sure, maybe on a 400 if the slugs are down the hole and you are at a higher altitude in cool air, like Denver. I knocked the upper rod bearings out of a 400 on 93 with an octane booster, so I found out first hand what happens. They also liked lots of ignition lead, the 455 I used to run them on liked 52 total, that is a lot. It would slow down almost a full second if I dropped the total timing to 38. Not the most efficient chamber design.
I have a set or ported 670s I am putting on the 70 GTO's 455. Why would I do that you ask? Well I have E85 available all the way back to Nebraska where I grew up, and I make my own ethanol fuel too, so I can run lots more compression. Why not run as much as you can get if your fuel can handle it?
If you have E85 available in your area, it is way cheaper than race gas, runs cooler, and is usually even cheaper than 87 pump gas. You can get a Holley already set up to run it, or you can send me a Qjet and I will convert it for you, provided you have E85 near you. So far I havent found any problems with running it in my 70, but you cant switch back and forth from E85 to gasoline with a carb, its one or the other. The only changes I made were an electric pump years before I started running E85 in it, and modifying the carb to flow a bit more fuel. The rest of the fuel system is stone stock from 1970.
If E85 isnt available near you,then find some 6X or 5C heads and run pump gas all day with them. In some states its hard to find, but as a way to make lots of power on the street for cheap, its worth the effort to run it. E85 is the only thing that comes out of a gas station pump that you can run with that compression.