That's the A pillar patch, or front pillar patch. You'll probably need the front cab supports and then the B pillar patch pieces (rear cab floor support, lower B pillar patch, rocker box reinforcement panels).
Here are a couple of threads that have some info on the surgery you'll be performing, if you haven't seen them already:
Replacement Truck floor in a Blazer
Project: Bolt One On
My '71 Jimmy Build Thread