You need to add horizontal braces on the top and bottom (where allowable for your intake) on the diagonal members of your 6 bar lift. 2x C-channel on the top, and 5x C- channel on the bottom preferably. Also be sure to add elastics(rubberbands) to each pair of your horizontal bar to aid in the lift capacity and it will also help stabilize your lift. These 2 things should give you the stabilization you need for a 6 bar lift.
I would highly recommend using long bolts and Nylock nuts instead of axles and collars in the lift’s intersecting joints. Though it doesn’t seem like a huge difference, it will save you from cutting axles to length, make the joints more secure, and make the mechanism more stable in general. I’ve never actually done a side-by-side comparison (I’ve used both, but they were on different lifts, so I can’t say with absolute scientific certainty because there was not a control), but in my experience, using long bolts instead of axles makes lifts much smoother, so long as the nut isn’t tightened to a great degree (use washers in between the nut and the bearing if necessary). Using bolts also reduces the profile of whatever goes through that joint, meaning that a bolt and a nut will take up less space than a small axle and two collars. It’s up to you whether you want to try it, but I believe it’s worth a shot.
I agree except that some people use shoulder screws instead of the regular screws (which is what I presume grammatically_uncorrect meant by “bolts”). The kind of shoulder screws Vex sells are not very good, but rule R7 allows you to buy shoulder screws from other vendors so long as they match the screw sizes and thread types that Vex sells.