It doesn't have to be level, it just needs to be flat and that concrete is flatter than the dip in your steel. Besides, you're using jackstands which are following the contour of the concrete anyway or at least in 1" increments if you're compensating with the adjustment in the jackstand.
I ended up slicing two more welds to release the stress in the rack but I didn't have to cut all the way through any pieces. I just had to tweak them a little and everything welded up really nicely. Still working so I will report back later.
98 3rz 4x4 5spd- Monstalined, 99 Talls, 4.30 E-locker, Extra Lights
In Progress:
Tundra/Rear Disc Brakes w/parking brake
Roof Rack/Rear Ladder
Sliders
1st Gen Rollbar Shelf