As this subject has been thoroughly discussed some time ago, however
there are a few reasons why the plate has cracked, one being a faulty
casting with blowholes within, and casting sand.  The plate might
never be satisfactorily repaired due to the faults.

For a while I worked in a machine tools shop making new, large centre
lathes and I came upon this problem, and it would break small drills
which took hours to remove and clean-up.

After the welding it would be good to anneal the plate: say, heat to
1500 degrees F. (blood-red heat) then allow to cool very _ very_ slowly.
And better then to cover and use dry heated sand, thus preventing and
retarding radiation.  This will allow the molecules of the structure to
contact uniformly and so leave a uniform stress throughout the plate.

What you are going to do is well worth a try.

Steve Bentley
Vancouver, British Columbia