Hi,
I believe you have all wrong the long ball screws, the plate attaches to ball screw mount and that moves the gantry.

They have to be adjustable -in all directions, like 3D / the mounts/ and the bracket has to be separate and allow adjustment vertically, oblong holes, after the leveling of rails /epoxy??/ .

Like you have drawn it, what would you do if you need to adjust the screw mount in direction towards the frame? I know you can file it but will be very difficult to adjust it.

IMHO but some may not agree, the correct procedure is this- to level the long rails, then mount then parallel, then mount the gantry , adjust the gantry in all directions. Then mount the gantry side plates. Then mount the ball screws via attaching the housing to the plate. Then move the gantry one end. Then attach the ball screw supports and if there are additional support plates- fix them via clamps. then move the gantry to the other end and repeat. Tick some dots with the welder to fix the additional plates. Then remove the ball screws and gantry and Then solder them and then paint all.
When all ready, the procedure is the same. you will have nice aligned parallel in all directions ball screws that will not need playing with steps in Mach3

Hence the need for additional support plates .

They can lower your ball screw length by the width of the gantry step, say 300mm each side. And also you can fit the motor there.

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Of course its up to you. You can do it as you originally intended, it will work but just leave say 3mm below the ball screw ends supports for adjustment . Repeat the procedure as described above and when you reach the moment you can shim them or even better shim and epoxy them.