308 springs

I'm getting my heads redone and need some information on these heads. I know the castings are a little thin and need to cut the spring pockets to get the spring pressure down a bit. What is the maximum I could get away with, the final seat pressure will be 140 closed and 320 open. How much cat I take off the spring pads safely. This is for a street motor (3:23 rear) that won't see much over 5500.