both have advantages and disadvantages.

Epoxy makes sence if your floor is level and machine is square=ish, to take out imperfections,

Shims give you a very high level of presision, but take time to get right

but , then i start thinking back to the days i had a go-cart circuit, and the jigs we built for straightening the frames, we could easly achive very high levels of precision, at complex angles, much more than you would need for rails.

So am thinking why not build a simple jig, into the top of the frame, both sides down the X ( 80mm or 100mm box ). a simple bolt pattern jig, Fix a bit of say 80-160-4, on that just leaving enough overhang to fit rails to, use the jig to get the 80-160 perfect, with a laser level and a few mirriors, you could get a very long X, very accurate, and make sure the opposite x was perfectly alligned.

I do get that this would be a waste for any machine with less then 1600mm X, but, you could be close to unlimited on X length andd get very accurate, with just a very basic tool kit.

My proposed build plan goes like this.

wield

Build 2 ends in Jig ( Y ) 1400mm
Build 2 sides in jig, ( X ) 2500mm , with Jig/bolt patterns

Assmeble square, by bolting together

Fit the 80-160 to the X, by adjusting down on the bolt/jig, to get it perfect as possible, using simple laser.

Then, bolt a bed, on, using you X to ensure your fitting the bed as close to perfectrly level with the x.