The chances of it being square when done are slim, treat it as sections and concentrate on getting each section as perfect as possible, whatever method you use there should be a method of tweaking the fit later - my plasma table is 1500 square, it is made up of two side frames bolted together with cross-tubes - i could shim the joints as needed which wasn't in the end but handy still.

The plasma was built in 2mm box section but carefully braced where needed to gain rigidity. I used about 21v / 140A and 0.8mm wire with a tri-mix gas (hobbyweld5) you need to get hot and fast, don't run the power low because it might warp.

Cant help with epoxy, never used it I'm afraid.