I'm building my first Pontiac to put in my 79 Formula (originally 301 powered), automatic with 2.73 gears. It is a 1975 400 with 5c heads that have 2.11" intake valves and 1.66" exhaust, screw-in studs and guide plates. These heads are in excellent condition. I also have a set of 1976 6x heads with same size valves, studs and plates, the problem is they were on a block that was left open for 2-3 years and got full of water, most of the valves and the guide plates are ruined and the castings are very rusty as well. I know I could use the valves etc from the 5c's but is it worth the hassle of all the extra cleanup work? The machine shop will have to charge me for the extra blasting and magnafluxing for the extra heads, so I would just as soon use the 5c's. This is going to be a mild 400 with an 068 cam, stock 4bbl intake and Q-jet with log manifolds and 2.25" duals. Any comments or suggestions? Has anyone flowed or dynoed a comparison like this? Thanks guys.  

The 5c heads have a bigger chamber than the 6x,114cc vs 101cc.
Mill the 5c about .006" to make them flat, nominal crushed thickness of head gasket .042", should push around 350hp on 93 octane. I did the same car with a 1975 400-4, 5c heads, .449" lift dur 282, 800cfm Q-Jet,(came with motor) I got 385hp.  

Thanks for your input,my 5C's are the 101cc chambers and rated at the same 7.7:1 cr that the later 6X's came with, also the Wallace Racing site lists the flow numbers as very similar. I plan to use them seeing as I can't afford Edelbrocks and don't want the hassle of trying to find better heads, I had enough trouble finding a decent usable 400! For now I am installing a 75 350 with 5C's, 4bbl and Crane 272 stick just for cruising.