Hi Ernie,
I am with Bruce forgo the flux and yes you need to either MIG weld or better would be TIG weld it. Another way I would go would be to just weld the top and bottoms not the faces where the pattern is and put it in a can for the welding. This would make for less welding and grinding, just be sure to put some WD-40 in to burn up the oxygen and wrap it in some used HT stainless foil so it won't weld to the can.
That should clear up your white lines and surface grinding the billets will make everything fit nice and tight for welding.