I have welded cast iron successfully many times - things like exhaust and
intake manifolds, lathe beds, drillpress platforms etc.  To avoid all the
hassle of preheating and postheating the cast iron, just use pure nickel
welding wire;  it has about the same coefficient of thermal expansion as
gray cast iron.  It is readily available commercially, and is sold for
this purpose.  Being soft, it is very easy to clean up the welded area
later with a file or grinder.  Where necessary use Bondo to fill any
pits that remain at the interface of the nickel and iron.  The real
question is does the plate really have to be welded?  It is normally under
heavy compression, but I suppose they may be some spots where there are
tension vectors. Is is buzzing or doing something unpleasant?  - Jack