What i think is it will be good if you read my second build log , look at signature. My advice is build the gantry like mine and make bed from 14cm UPN like i did , there is nothing cheaper and stronger than that. When welded to the bed, it acts like 10cm solid block of steel...

If you build the gantry exactly 1:1 like i did it, it will weight around 300 kg and will be ridiculously stiff . You should worry more about your Z i think. About the overhang and so on... Of course if you encase the spindle like i did, would be the best action. The way i did my gantry and my Z= same like double gantry and spindle in the middle, if not even better.