Which year 4200 are you working with?
If your starting with an 2002 -2005 - try and find a 2006 up head/cam cover/ignition coils/VVT/exhaust manifold.
In either case port the head - especially on the exhaust side. If starting with an 2002-2005 replace the valves/seats with ones from the 2006 up head - they are 1mm bigger OD.
The intake port just needs a blend on the short side to eliminate the sharp edge (once the heads apart -its sticks out like a sore thumb) and blend in the pocket. The exhaust side needs work, count on 3 hours/port. From the seat to the port exit needs work. GM did this for emissions. Once ported the exhaust flow will increase by almost 45% on a late head.
If your going turbo and using the stock internals:
Open the top ring gap to about 0.016/0.019".
Assemble the rods with new 10 under bearings and accurately measure the inside bearing dia. Get the crank turned 10 under for 0.018/0.0021 rod bearing clearance. The mains are fine at stock clearance.
Shim the stock oil pump relief spring about 3/32".
Route oil from on the oil filter outlet to the main oil gallery via the external oil plugs on the pass. side of the block. I have the needed -8 fittings to do this.
