Re: Making a factory action "custom"
I understand what your getting at. How the heat treat of the action will change from welding the extenson to the receiver would be my main concern.
I do think it's a good idea, even if you had to reheat treat it. What I would do, is bevel both edges and weld it all the way around the action.
I recently read the new larger chamberings in the Savage rifles have a near 1 1/8" diameter thread like some of the customs do, still short though.