I am not recommending amtex for porting, just their repair facilities. Cheaper isn't always the best route to go. If I were you, I would ask about what kind of equipment each place has, and what methods they use. You would be surprised how many people think a belt sander is the proper tool for adding clearance to rods, or removing clearance from mains.

In my opinion, if you are looking for someone to do general machine work for cylinder heads, Clearwater Cylinder Head is a good option. Don't laugh, they have a Serdi guide and seat machine. Because they do nothing but cylinder heads, they have decent specialized tools for doing just that. Pricing and turnaround is reasonable too. Just be specific with what you want.

As for porting (and valve job), HPS, Precision Porting, Walter, all are good options.

Brian