The 58cc chamber sounds nice for compression, but I wouldn't use them on a 400 unless it has dish pistons and a mild cam like a CompCam 268H. I also wouldn't use them on the 400 unless they have at least the 1.84 intake valve. I have heard that a 1.94 intake valve will be shrouded in the 305 heads. But I don't know if that is on a 305 block or all blocks or what. Maybe sombody else can shead some light on that for you.

You would be better off with a set of 333881, 333882, or 333883's. But, look at it this way, I have a set of 883's on my 355, I spent $750 on them installing 2.02/1.60 stainless valves, 1.44 inch dual valve springs, new guides, hard seats, 7/16 studs, guide plates, and I had them drilled to work with 1.6 ratio rockers. And I still have a set of heads with stock ports. Granted the 883's do have a port around 165-170cc's, but for the same money I could have had a set of World Products heads. The only reason I used the 883's is becouse they came with the motor, and the motor was out of my friend's first car and I wanted to keep it whole when I dropped it into my Firebird.