My thoughts and suggestions
The stand looks to be younger than the lathe. In fact, it looks a lot like the stand under my Enco 13x40 from roughly 2005. I consider my stand to be fairly flimsy, based on looks and sheet metal thickness. (I have plans to build a proper stiff stand… someday.). Maybe yours is heavier stuff.
I like those angle iron rails with adjustable feet. As long as you don’t hurt yourself on those sharp tips.
I’d use the adjustable feet to “flatten the floor”. Use a level to make the frame rails parallel to each other and in effect place the lathe stand on a flat floor. Once “flat” I’d stop messing with the adjustable feet.
Then I’d do the 2-collar test, etc., and shim the tailstock end of the lathe bed in the usual manner. That means pulling out the silicon you put in there, but that’s easy to replace after the machine cuts true.