Jim someone cut that stand down and welded those tabs on it. Here is a couple quick pics of my G4003G and my 2 cents worth on this topic. First the stands are far from precision, look at the left stand below my headstock rests on, its like trapezoid welding project gone bad. Next consider is the bottom of the lathe casting actually ground flat and true head stock and tailstock on the same plane, unlikely. That's why the two stands are independent of each other. If you suspended the lathe in the air with the stand bolted to it I'd rather imagine the two stands going in several different directions from one another. Remember a tiny bit out at the bottom of the lathe casting is a lot out by the time you get to the bottom of the stand.
Okay so lets assume the stands are wonkers like mine are. I just recently removed the leveling feet from my G4003G, as you can see in the first pick if you bolt level feet on this considerably shortens the footprint of the stand front to back which is already too shallow imo and tippy.
Here's what I did, I set mine on the floor and leveled all 8 corners of the two stands independently using my Starrett precision machinist level making sure all 8 corners were in contact with the floor and not floating above it. No two corners were the same in terms of shims and my garage floor is flat, its new construction only 3 years old. I have more shims in the front of mine because my garage floor like many are sloped towards the door for drainage, e.g. building code.
If the OP really wants to bolt the lathe to the floor what I would do is set the lathe flat on the floor and shim it level as a first step. Then take a couple lengths of steel tubing, weld or bolt one to the outside bottom of each stand, then bolt that to the floor. I'd cut them a few inches longer than the depth of the stand to give it some more depth and stability front to back.
Parting note, unless you are pretty tall 6 foot plus you don't want to add any height to the lathe its already too high as I recall its 4 inches higher than the RML 16x40 lathe I have been looking at.