MY THOUGHTS EXACTLY. THE OP STATED ITS WELDED FRONT REAR AN SIDES SO ITS MORE THAN JUST WELDING TO RIVET SHANKS.The issue as I see it would be that the hardened area around the locking lugs would lose their temper and unpredictable results would happen. Premature headspace issues thanks to softer metal in that area or worse. But this is speculation and assumes so much welding was done that the heat treat of the trunion is lost.
I HAD A TRUNION HERE FROM A BOTCHED BUILD THAT WAS WELDED EVER WERE THE RECIVER TOUCHED AN THE THING WAS SCARY TO LOOK AT.