A machine shop is best IMO. That is because our heads typically need a valve job to recut them and get them to seal good. It's almost a given from the experience I have had with 5-8 sets of heads. Also it is good to have them check the guides for slop etc. Magnafluxing (aka checking for cracks) is also huge. Often we can't see the cracks without this process and 3800 heads are known for cracks between the valves.
My typical work up is
Disassembly
Hot tank (cleaning)
Magnaflux
Valve job
Consider having the spring seats cut to accomodate the springs you are using. (stock height is around 1.72xx", with modded retainers 1.78xx"
Consider having the valve guides cut to accomodate more lift.
Paint and reassembly (usually with new springs/retainers)
