I didn't see whether you were going to mount the beam on a work bench or make a stand. If making a stand, weld four legs to the beam and add leveling feet (bolts and nuts) to the bottom. Then you could drill and tap the lathe mounting feet and use longer than necessary bolts. Sandwich the lathe mounts between two nuts and use that arrangement for your fine adjusting and truing. That way you take the uneven surface of the beam out of the equation. Once trued, you can always pour the feet in epoxy grout for added insurance.
I've had my 12" Atlas mounted to a double layer of plywood since 1985. It works but I have to be careful, and I've planned to build a steel mount for it for decades. I would use the beam with a few considerations mentioned above.