I cant see any reason to have the plywood when bolting to a concrete floor. Am I missing something?
I would get rid of the plywood, drill four holes into the concrete and dyna-bolt (or similar) the stand to the concrete.
Before tightening the bolts down, slip a feeler gauge under each of the four bolt down areas and if necessary, put shim under the stand to take up any gap between it and the concrete. This way, you are not twisting the stand when bolting it down, and hopefully avoiding twisting the actual lathe bed in turn.
Final levelling can be done with shims or raising blocks or whatever you want, directly between the lathe itself and the top surface of the stand.
Don't put a bracket to the wall of your workshop from the lathe stand. The wall will then reverberate all the noise and vibration of the lathe working and make it ten times worse.