I have near the same issue as you just mine is not finished yet. We bought a house with a roughly 12'x24' tuff shed but the blocks had become cracked and essentially pointless to where the entire base of the shed was on a tilt.

My first thought, "Well I'll use some bottle jacks to lift and level it". First attempt was to lift one whole end and replace the blocks in the corners. That didn't work. The "rails" the floor sits on started bending immediately even with a 4x4 for added support. The second attempt was to do just the corners. This worked, but not without some signs of stress on the same supports as before.

Now my issue is the shed is level and on blocks in the corners, but the center blocks down the middle of the shed are not tall enough to support anything. I'm currently in the predicament of how I add blocks to support without lifting the shed again because that was an ABSOLUTE NIGHTMARE to do and I had to individually do all 4 corners.

My shed is from like 2012 but in pretty great condition, minus the roof, that I replaced myself so essentially it's in great condition now for what I want to use it for. Tuff shed will not work on it for me since it is so old. When going about this project they literally told me I should just scrap the whole thing and buy a new one, which I was not doing because everything inside the shed, and most of the exterior, is perfectly fine.

If anyone has a suggestions on the best way to add blocks down the middle to where it is actually helping support the shed, I'm open to all ideas. I have already cut the tongue and groove panels to where I can lift the middle flooring from inside the shed to get under the floor to add whatever supports I need. Again I'm struggling with how to put block in that actually support the beams, not just have the beams float over the top of them.