Spread the bread on a baking sheet and bake until slightly crisp (like stale bread) but not toasted, 20 to 25 minutes. Allow to cool.
Preheat a grill to medium-high.
Whisk together 2 tablespoons olive oil, the garlic, red pepper flakes and a pinch of salt in a large bowl. Add the shrimp and toss. Set aside while you assemble the salad.
Whisk the vinegar and remaining 1/2 cup olive oil in another large bowl. Add the tomatoes and onion, then season with 1/2 teaspoon salt. Add the olives, capers, bread cubes and basil. Toss and let sit at room temperature while you grill the shrimp.
Grill the shrimp, turning once, until just opaque, 3 to 4 minutes. Once the shrimp are cooked, taste a bread cube. If it's a bit dry, drizzle the salad with a few tablespoons of water and toss. Add the grilled shrimp to the salad and toss once more before serving.