If its warped they have to straighten it first, before machining.
I have recently bought a 323 and it had a warped head(alloy), it was warped 3mm in the middle, the head is only 18inches long. Thats a big bend.
They heated it in spots with the Oxy tourch and the head sort of just poped back straight, then they only had to machine 1/4 of a mm of.
It cost me $200 for thestraighten and recon.