In my very humble opinion, a mill comprising of a fully welded and braced steel plate box section framed body would make a far more interesting design,
Handlewanker, christofer
If you welding together, using thin plate (maybe <0.5 in) you need to stress relieve.

you have any trouble with warping or inaccuracy of the rail supports
That is true, I experienced like that on my previous build. Use thick plate or steel block and bolted together is cheaper and faster way and alot heavier.