This was on a pole barn with the horizontal boards 3 foot on center so it was relatively straight forward except the universal wall brackets don't match up to the CtoC.
So I did fabbed up some 2" aluminum angle 1/4" wall to tie the upper and lower boards together and give it some support between the lowest board and one further up. Pretty stiff that way. The AC brackets were then attached to the Aluminum angles.
The angle supports also gave me the option of how high off the ground I wanted it to bee
How high? 1 ton Pickup tailgate high. That was the easiest way to get it up there by myself. Backed up and slid it over. Lots of room underneath for snow drifts, etc.
In your case, having had a metal/metal building in the past I feel your pain.
I would think a sub wall, glued, screwed, sheeted and then screwed to the floor would do the trick. Even better if it can be tied off a nearby column. And then you have a place to hang stuff on the inside. win/win?
Edit: Things I learned along the way.
IF you use one of the 2 ton and up units which are 150 lbs, make your own universal supports. I went to an ungodly amount of trouble to mount everything dead level and plumb and it was ...until I mounted the unit.
The sheet metal universal brackets sagged outward from the building and down on the side where the compressor is located. Needless to say I was miffed. I had to shim the snot out of it to get back to level.
Yes, the wall support and angle mods were still plumb.
Those brackets might be fine for the 1 tonners but no way would I buy a set again.
