Best bet for pistons is TRW, they're about the only company that still lists forged pistons (just like OE) for the 235. As far as milling the head, you shouldn't have to do anything with the rockers, valve seats, or spring seats. The combustion chamber is pretty big on the 235, and the exhaust valce not only sits rather high up in the chamber, but is also canted as bit.
Not much to be gained by porting, either. The intake ports are already matched up, by virtue of the alignment rings. One thing to watch for, make sure you use a 235 intake. The 216 uses smaller port rings, and will be difficult to get proper alignment and sealing.
