03A3 rear sight dovetail restoration

[vintage hunter]

Don't use cold rolled steel as it will buckel from the heat,

So will the hot rolled, trust me. If I'm understanding correctly your suggesting plug welding the piece to be added via the 1/4'' hole in the center, right? This will cause the add on piece to curl up on the ends as the weld cools. Weld has enormous drawing strength. I've seen it pull grade 8 bolts until they break. Also, the rear base screw has to go somewhere around or about the center and drilling/tapping through weld is often problematic.
My suggestion is to attach the piece being grafted with two countersunk screws and low temp soft solder, the way No.4T scope pads are attached. You could either machine the dovetail to it's finished dimensions, use the base to locate it correctly and solder in place then add the two cs screws or start with a blank and screw/solder it first then mill the dovetail last as suggested above.