Well, some of those "cut and weld" receivers did crack at the welds, no matter what anyone says. The location of the weld meant that none (AFIAK) caused any injury or catastrophic failure of the receiver, but to say that none ever cracked is not true.
The receiver heel is not intended to stop op rod travel; that is done at the front of the receiver. The heel allows a place for the bolt to stop when play between the op rod and the bolt allows the bolt to keep moving after the op rod stops. Normally, there is little force applied at that point, but the heels did crack (just like that) when firing rifle grenades before they developed the valve type gas cylinder lock screw and the grenade launchers that open the valve. Subsequently, the heels of new receivers were annealed in molten lead, and some were annealed during rebuilds; probably that one was not treated.
I guess I am on the fence on this one. I agree that continued firing of the rifle is unlikely to cause any further problem, even if the cracks are not welded, and that welding them will do no harm. Yet, on general principles, I don't like the idea of continued use of that receiver unless absolutely necessary. It is not a matter of life or death, of needing the rifle to defend home and homeland, and there are other M1 rifles available at reasonable cost.