1950 ES2 valve springs

I'm in the process of overhauling the cylinder head on my 1950 ES2 (cast iron head). I've noticed that the valve springs appear a little soft, i.e. lacking stiffness. Thought I had better check them out before re-assembly of the head. Can anyone let me know what the stiffness should be or what the free-length of these springs should be (the wire diameter and number of coils would also be helpful)? I can then check mine to see if they are 'OK'.

Thanks

Gordon