No welding, it will probably show after finishing if blued. Countersink the screw holes and fit screws with a proud head that fits the countersink. Peen lightly to swage them out to a tight fit. File and polish and then reblue. The countersink and the screw having a head is imperative, otherwise you’ll have half a thread showing at each hole. I can take pics of a receiver I’ve filled scope base holes on and you won’t be able to tell it was ever drilled.
