smiths talk about welding the firing pin hole closed and then redrilling it in true dead center, on some accurate rifle actions
Only if they are crazy.
Welding on a bolt near the lugs is a really bad idea.
A bolt with rear locking lugs might be OK (though they are not normally considered great candidates for high accuracy), but not a bolt with front locking lugs.
Even the possibility of altering the heat treat in the bolt and locking lugs is not worth welding near the lugs, and the bolt face is near the lugs on a front locking bolt.
The bolt face is bored and bushed, then a new firing pin hole drilled.
The bushing can be a press fit to eliminate the additional work of threading (and the loss in strength compared to solid metal).
The bushing normally has a larger diameter at the firing pin face, then steps down at the rear.
This allows the flange on the bushing to transfer more of the load to the machined back bolt face.