Look in the corner of the heads like Itooldyouso said, right beside the edge of the valve cover in the corner you should be able to see a 2 or a 4 on the head.
You don't have to take anything apart to see it.
As far as I know all the Cleveland Heads were the same, I never heard that the 400 had a differant head. I though all the 351M's and the 400M's came with a 2.05 intake valve in 2V heads.
Though there was 2 differant 4V heads, 2 Differant intake Valves. 2,23, 2.19
I think the 2.19 was on the Boss 302's as well.
Kurt