Just pull your engine control computer and disconnect the electrics before you weld. Sometimes they can get zapped by welding.

Personally I'd take a plate of steel, pound it into shape, temper it and weld it to the under side. I'd clean up all the rusted metal in the area, weld it in on the outside being careful to have the mount height the same as original. Then I'd paint it with a rust blocker like Chassis Saver or POR-15. If you are doing all 4 mounts, then you can lift it equally at all 4. Bumper may no longer line up the best.

-- Bryan