The casting number is on the bottom side of the intake runners. Real nice and convenient, eh? If they're still on the motor, you'll need to remove the intake manifold and use a mirror to see them. The date code is under the valve cover between two of the valves.
Of course, if you want to know what they are -- two or four barrel -- there is a large 2 or 4 on the upper corners outside of the valve cover. If there is nothing there, they are generally 351M/400 heads.
trust me , me and jeff giffen here we got 351c`s w/ 351m heads and ya need extra gasketmaker in the front n rear cause of the taller height. But that will all change now that at the end of the year im taking my vert off the road and yanking the 351c/m and gonna build a nice strong 351c 4v engine for it. But it will be a while like i said