You can weld the back end of the bolt without any worries about loosing strength at the front where all the punishment is delt out , I've done two of mine and haven't had any problems. All I did was wrap the bolt head with clod wet rags , the heat never traveled any farther that about half way the bolt head was still cool to the touch.
I saw a Rem 700 once at a gun show that had the recoil lug and bolt handle welded on , it appeared to be eith "stick" or mig welded , it was built way up and tried to be gound to look decient , it was awfull , you could still see some slag down in the weld.If they diden't have enough since to weld it right I wonder how hot they got the front of the action and blot head , all I could do was shake my head and laugh
Heli-Arch is the only way to go.