Browsed through this pretty fast. On the discussion on whether it was CI or steel the welder should have known as soon as he started grinding the V. Difference is sparks is notable. Not familiar with the tractor; but, is the bell housing part of the tractors structural frame supporting the running gear? Older tractors were made that way. Now frames are structural steel shapes due to the cost of those heavy castings today.
Cast iron in place and possible under tensions I would do the repair with bronze welding either torch or stick electrode. It is more malleable under stress than nickle rod. I have used both successfully.
Ron