Looks good. I would probably extend the gantry uprights down to the floor so the more area would be available for welding and increase rigidity. the vertical rails could then drop down and increase the bearing spacing a bit more.

I think golden ratio is 2:1 so if the y axis is 800mm wide then you need 400mm bearing spacing. as you pointed out one change can affect another and you will need to look at flex in the mounting plates and possible bracing,


Do you think the gantry uprights need bracing as well? looks quite tall

It may have already been mentioned but how to plan to keep all the surfaces perfectly aligned whilst welding and given the surface tolerance on steel c section?