Hi Routercnc,
Yeah I knew the frame would bend a lot with welding so wanted to allow for alignment of the gantry. I've got M16 bolts in 18mm hole, and have some jacking screws between the gantry and the frame to shift it about before tightening the M16 up hard. This actually worked really well.

You're right about the plates at the side. This would be a bit harder to do now because I've built an enclosure around the thing that I'll post some pics of later, but I might go back and add the plates at least on either the inside or the outside of the vertical tubes at a later stage if it seems like it needs it 🙄