Yes, I have boxed a set of lowers and a set of uppers. The lowers have been on the car for a year or so, work fine. I still put my traction bars back on, so I don't have a direct comparison of improvements, but with both it lauches better than with neither, and with bars, it is better than just the boxed LCAS, from what I remember.
I boxed the uppers, but I have not gotten around to installing them. Same for some poly bushings in the lowers.
Total cost was about $20 for some 1/8 in flat steel, and I bought a new roll of welding wire. Made some cardboard templates, cut them out with a torch, ground to fit, and welded them in. I welded in short beads, alternating sides, ends, etc. I cut and welded some new swaybar brackets also. You could leave the originals in place and just cut a slot for them, but you could never remove them once you weld the plate on. My new brackets seem to also work fine. Painted when done. Be careful not to overheat the bushings when welding.
They seemed to fit fine.
It now does 1.88 to 1.95 sixty foots, with all stock 1980 rubber bushings still in place.