Instead of the full width, 6" thick slab can you simply pour a thicker footing for each mount point. Say 30 inches square and a foot thick and tied into the rest of the floor with a lot of re-bar? you might save a lot of concrete costs. Have a look at the lift floor requirements.
I have a friend in calif who did just that in an existing garage. He cut out the existing 4-inch thick floor in two places 3X3 feet with a concrete saw, dug the holes two feet deep, drilled sideways into the existing concrete, and installed a lot of rebar in the holes as well as tied into the existing slab. This was for a two-poster. It worked very well and saved demoing all the garage floor and re-pouring it. The lift company, as well as the county inspector, was ok with it. Just a thought.
I would put the stairs on the back wall also. Maybe build some cabinets under them. Or you could place the side entrance door under them.