Most lifts with the top plate do not rely on that to keep the columns from tipping inward, it is for routing the hydraulic lines and guide cables, and keeping the floor clear. I just installed a fixed post, low ceiling lift from DanMar, it has a plate on the floor between the posts, but again, it does not tie the columns together, it just covers the cables and lines. In my case I framed my fllor myself with the knowledge I would be installing a lift, so I made sure that the locations for the posts was deeper than th erest of the pad, and that I put in plenty of rebar and mesh to make it stronger yet.
If you are concerned about the strength of your concrete, get a couple of anchor bolts, drill test holes close by your installation location, install them, then see how much torque they can take before they pull out.
