It is not suprising that some old rocker boxes have distortion given the twisting forces of the valve gear exerts on it and the abuse many will have suffered over the years.
More concerning for the light alloy heads is that the joint faces between head and box have slightly different shapes. Compare the front joint of the inlet side of the box with the head and you will see that it gets quite narrow where the surfaces dont quite match. The try a commercial gasket and you will find that it does not quite match either head or box.
In an attempt to stem the leak at this joint, SRM, at my request, built up the inner edge of the box and machined the surface flat. This has widened the joint surface and made a better match between box and head and improved the sealing of that joint.
A leaking rocker box, particularly on alloy heads, is nothing new. See the interesting bulletin issued by Hap Alzina in the USA in 1959.