Recommend that you remove everything in the vicinity of the cross member. If you can remove the fenders and inner fenders then do so. Measure the height location of the core support for later reference. You need to put it back in the same exact location as before it was removed. Temporarily weld braces across the frame horns top and bottom to assure movement is minimized. Then remove the old cross member and clean up the area to which the new one will be attached. Make good welds in all of the accessible areas keeping in mind that 100% weldment is not easily attained or necessary, but do as much as you can. Remove the temporary braces and grind the temporary welds smooth. Reassemble and (viola!) you are done.
Good luck.