There are two machinists I can highly recommend.
First is Mark DeGroff's Cylinder Heads in Northridge.
Address: 18736 Parthenia St, Northridge, CA 91324
Phone:(818 701-5274
Second is Costa Mesa R&D in Costa Mesa.
Address: 123 Monte Vista Ave, Costa Mesa, CA 92627
Phone:(949) 631-6376
Both shops do great work. Costa Mesa R&D recently did my engine's bottom end, and I have used Mark for several different engines over the years.
There is nothing particularly special about the slant 6, it's up to the engine builder (that's you) to specify the type of work needed and the precision you need. These guys can help you figure it out though.