Note that the hardness of the E/G in this type of design is less critical, because the inserts provide most of the support for the linear rails. A lathe would be a particularly good application because nothing sits on the E/G but the rails. Hence the job of the E/G is then most to be stiff rather than flat. You could build a slant bed design pretty easily too, if you so desired.
It's worth taking some time to carefully think through how to get the headstock and spindle properly aligned with the bed rails and similarly dampened with E/G.
Cheers,
BW
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