Sorry man,
I just looked in the LMC catalog and I dont see that part.
It should be called a "B" pillar or lock pillar, GM part# is 347115(discontinued), try napa or another supplier in your area. If you can't get the part then you'll have to cut out the exising one, beyond the area thats already been welded, and just make a new piece of metal to fit in the hole.It would be best to weld a backing plate in so you have at least 1" of overlap for strength.