If you mill the head the pushrods are now too short by the amount that was removed from the head . I have no use for cast pistons . the same for cast rings . Cheap crap IMO.
Pushrods may, or may not be too long. It depends on how much milling, and how the tolerances add up. Also depends on the cam base circle. I have run stock push rods with a head milled .125, and had to get shorter pushrods with the head milled .100.
As for cast pistons, unless you are building a pretty stout race slant, cast are fine. I have stock cast pistons in most of my engines. Cast rings are good on used bores (they will seat quicker, but not last as long). I like moly rings, on a new bore/hone.
PS: forged pistons for a slant will cost about $500 or more per set. cast pistons are less then $100/set.