Clamp it in a piece of heavy angle iron, lining up threads as close as you can - then grind out the accessible area - weld, then turn over and do the same for the other side, this will be the best way to get the threads aligned and stay aligned during welding - you may have to tweek it a touch to straighten after all welding is done, but it shouldn't be much. Then touch up the threads with an angle grinder with a cut off wheel to get into the threads/ or file etc.