I had a plate heli-arc welded one time by someone who advertised that
service in the Piano Technicians Guild Journal, who has a good reputa-
tion throughout the trade, and who had the credentials for that job.
After the plate was returned to me, I thought that I very gradually
brought the pitch up, as I was warned to do.  But the crack reappeared,
as he said it might, although there was a 90% chance of success.

It is a big deal to do the job well.  The entire plate must be super-
heated in a giant oven before the welding is done.  I paid $400 then,
probably $600-700 now.  Maybe the technology and techniques are better
now.  Many welders guarantee their work, but should the repair fail,
you have to undo the stringing, go to larger tuning pins, maybe a new
pinblock, and now the cast iron is more brittle in the cracked area
from the previous welding.

I would do both, weld and reinforce.  Factors to consider are: the
location of the crack, how much the structural integrity is compro-
mised, the amount of string tension that area is under, and how much
load a brace (metal plates and screws/bolts) can carry.  I would call
in a piano technician with an engineering background to look at it.  I
can’t answer your question directly.  I am not an expert, but I have a
lot of experience in this area.

Bill Maguire