Need to develop a tapered spacer for under a car lift on an angled concrete slab
Isherwood has good points about the capacity of the floor slab, you need to confirm what you have. A standard 4&quot; slab may not hold up to the rigors of the lift. If the install guide says differently or you know you have a  thicker slab that can take it, then carry on...
Non shrink grout is typically used for this type of condition. Pack it fully by pushing it through to the other side of the form so you can see it bulging up against the form you have set about 1&quot; maybe more away from the metal base. Keep adding until the area is packed. It seems you may be able to use the butt end of a 1X4 to pack it in, leaving one side of the form off to pack by. The last side will not need the form. Just mound it up to keep it from &quot;slumping back&quot; from the underside of the metal base. The forms you make would need to be the height of the gap under the base, it can be higher, so using 2X4s or 2X2s are a good idea.
Once the grout has gotten its initial set, were you can slice off a section of the mound and it does not lose its shape, remove the forms and slope the  grout to a 45 degree angle from the metal base to the floor.