Sometimes even the most experienced shops can't weld a cracked aluminum head successfully. It has a lot to do with the casting itself. Cast aluminum is very porous and difficult to weld. Add to that the contamination that may be deep inside the crack and it becomes more difficult. I think when you add it all up, with the machining cost, if you do get it to weld successfully, you might as well buy a new head. I think Clearwater sells them for around $400 with valve train installed. Ready to bolt on, with a warranty.