They contend that the combination of aluminum valve covers and cast iron heads dictates a cork valve cover gasket for optimal fitment and to prevent oil leaks?
Hmm. The last three Cleveland engines I built used rubber or silicone valve cover gaskets in the setting of aluminum valve covers and cast iron heads. None of them leak. My advice is to respectfully disregard the supplier's recommendations.
I would suggest surveying the valve cover gasket choices on Summit or JEGS or maybe even TMeyer and pick a rubber or silicone gasket. I seem to recall Mr. Gasket and Victor Reinz have them. I think FelPro may also offer a rubber option.
Warmest regards, Chuck Engles