[CarveOne]

I agree with MrWild's angle iron comment.

Try it as it is initially, but if it becomes a problem, add a piece of the same plate that you used at the lower end of the angle irons to extend the plate upwards to the top of the gantry. Weld it to the lower plate and bolt or tack weld it to the angle iron every few inches higher up. (Or make new one piece plates.) You could taper the extended plates if you wish to.

Another option is to replace the angle iron upright with a piece of the same box tube that you used on the base. You could even use the upright plate in addition to the box tube if you choose to bolt the upright plate in my first suggestion.

CarveOne