Dean is right, if you expect perfect finish on aluminum you will have to overbuild like mine build. Ok maybe not so much but simplest will be 2x 100x100x3 box section welded together. Or something in the same range of strength at least. But also have to be fast so you can do HSM with light fast passes. But honestly i dont see how over that span you will go under 100kg gantry at least. My gantry is 180 kg for example with all mounted.

Dont you have now a large machine to do that stuff??