Without flattening out the plate before machining on it, you will wind up with a plate that is about 1/2 inch thick.

You are going to have to heat and press flat before machining to true flat. If you don't heat to ''relax'' it, it will turn into a pretzel when you machine on it. You need to find someone with an annealing oven large enough for the piece, and also has a large platen press that will take the piece, or at a minimum a large H-frame press that the piece will fit in.