These are ATF instructions sent to investment grade firearms:
3. Cut away the front corner of the trigger housing where the automatic sear is mounted, .410-inch from the bottom and .540 inch from the front towards the rear.
4. A new hole has to be drilled to accommodate the trigger spring. The hole dimension is .200 inch drill that hole .165 inch from the front of the new corner and .250 from the bottom of the new corner. These dimensions are for the center of the hole.
What they really want is for the original hole to be completely cut away and not just filled in. If you just weld it up and relocate it, you'd likely be just fine, but I think they prefer it to be completely removed. The converted SEF packs I have all have the old hole cut completely away.