If it is cracked right through in the location you indicated on the thumbnail, then it is basically scrap. USGI receivers are made from forged 4140 Cr-Mo steel, and for any welding at all you have to use pre-heat. The only area recommended to weld (TIG) on a receiver is a repair of broken ears on the dovetail cut for the rear sights.

USGI receivers have a Rockwell C hardness spec of 38-45, and the high temperatures associated with welding will significantly reduce the strength in the welded area (called heat affected zone or HAZ). Your particular cracked area is a high stress region, right behind the bolt locking lug.

Brazing will not reduce the hardness as much, but the brazing filler metal (brass or bronze based) doesn't have the strength required.

Retired metallurgist.