The big spot is probably the engine tune up label.
https://www.gm.com/content/dam/compa...olet-Truck.pdf
I don't see anything in the generic info packet about the tune up labels so your best bet there is maybe somebody with a 305 can post theirs up.
I don't recall anything going in the smaller location. Perhaps it was a local shop oil change record or something of the sort.
I also don't recall anything going in those two holes. The radiator supports are used across a lot of applications and need to support every conceivable option, so sometimes there are holes and features that are not needed on your specific truck.
K