Drilling the old spot welds out means that the new panel can be attached to the exact same places as the one removed, simply by plug welding through the new part, you wouldn't have holes left if you did it that way.
All you have to do is mark the positions on the new part where the holes need to be, then go along with your torch and weld, using the panel as the filler. You'll need an absolute minimum of rods this way or, if you use MIG, wire. Best of all, the seam will look exactly like a line of original spot welds.
I'd drill through and modify by tacking hank nuts to the underside of the wing rail so that the wing could be bolted on instead, there's no need for it to be welded, since that huge brace from the scuttle to the suspension pickup point is more than up to the job and it would make for easier future maintenance of the headlamp panels and eyebrows.