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Ideally, you want to remove as little as possible so you don't significantly alter the geometry of the chamber. A thicker HG can make up for a significant amount of removed material but you're still limited due to certain conditions such as the cutting down the quench area or cutting a block to the point that the piston crown meets or passes the allowable height with relation to the block deck. Your machinist should help you determine how to get the head in the proper shape (if possible). There is the possibility that it could be lapped by yourself with a honing plate and diamond lapping compound and then you could have them measure the straightness for you... It'll take a while but it'll get it as smooth as it much be for a MHG with the minimal removal of material. A lapping plate will run you around $100 and $20 for the paste... it's a good deal of work too, so if they can actually machine it it may be worthwhile. When you get the block (and possibly head) cut the must be as smooth as glass too. Where a composite HG needs the texture it's a killer for a MHG.