It all comes down to a personal decision about how much risk you are willing to accept. When dealing with a 50,000 PSI pressure bloom a few inches in front of one's face it's worth thinking over carefully before putting your faith in a welded piece of steel which is stressed by being hammered by the bolt in recoil. I once had a fellow show me a Garand receiver which was cracked in the front ring and tell me he thought it would still be OK if he used plumbers tape to "tighten up the barrel". I advised him against this, but he thought he might try it anyway. I never found out how it worked out for him.
Back in the late '50s/early 60's folks were welding de-milled Garand receivers which had been cut in the middle part of the receiver to the rear of the locking lugs seats. The lug seats were still OK in terms of hardness, but the peril, in addition to the questionable quality of the weld, was in trying to mate mis-matched front and rear portions to get the overall fit back to original specs. One of the main faults with this was changing the angle of the rear receiver bridge to the extent that this no longer provided the safety feature of keeping the firing pin to the rear before the bolt was fully closed. For more about re-welded receivers see Kuhnhausen's excellent Shop manual on the M1 Garand and M14 rifles. It is the definitive technical reference on these rifles and includes everything one wants to know about inspection, repair, re-building, and maintaining them.