Now that the paint has dried, here's a picture of the fiberglass grille block.
The upper block is lumpy, which I can fix with Bondo or a do-over. But I'm very happy with the looks of the lower grille block. It just needs to be feathered in, because it's raised a few mm from the rest of the bumper.
Btw, I painted the front of my car "almost-silver". It's not perfect, but I'm happy with it, especially since this is my first attempt at painting a car.
I've tested the grille block at 60°F @ 74mph, and 75°F @ 60mph. The coolant temp stays put at 195°F, which means I should block more of the grille. I think I should block everything to the driver's side of the license plate, including that odd, large radius scoop whose original purpose appears to be to make the apparent size of the grille larger than the radiator.