That with the long screws holding the plate is great idea, thanks for sharing the photos. I will do the same, as i don't have long clamps.

All looks very good!

I fully welded the gantry on the machine i finished and spot welded the Z. Now that you have all in place why don't you repeat the spot welding until all spot welds are very close?
If it was me, i will fully weld it, though it seems that will work both ways. Welding precisely takes a lot of time , i know. I made 5cm stitches, and took care that the temperature of the gantry not to rise above 60 degrees. Used an infrared thermometer. And had to wait a lot for the metal to cool.

You were lucky if your long plate was straight. I had to hammer down to rectify some of the laser cut stuff. Good that i have very big plastic head hammers.