what can i expect 318 with x heads

Consider adjustable rockers, too. If you mill that much, you will likely need them. Lifter preload with a performance engine should be somewhat less than a stocker. I like to run no more than .030" preload and like it around .020". It will likely be much more than that with milled heads and stock valve gear. You can use shims under the stock shafts, but it's not as accurate as adjustable valve gear.