Looking good so far. Looks like there is some adjustment on the upper gantry (I see some plates and holes) ? That will help with alignment.

For the rear uprights which are near each other - wonder if it would be worth plating the outside and inside of them to form one larger box, especially if overlapped down onto the main frame. These form the stiffness of the gantry support, so worth a thought for not much effort ?
Click image for larger version.  Name: plates.jpg  Views: 200  Size: 97.7 KB  ID: 30604

Keep the photos coming, looks like you are well kitted out to get it done.