Preheat grill to medium-high (400°F to 450°F). Place a 12-inch cast-iron skillet or grill pan on grill grates. Working in 2 batches, add bacon to skillet. Cook, turning occasionally, until crisp, 7 to 8 minutes per batch. Transfer bacon to a plate lined with paper towels; let cool 2 minutes. Break each bacon slice in half. Remove skillet from grill; reserve 4 tablespoons drippings.

Brush reserved drippings evenly onto 1 side of bread slices. Place bread, greased sides down, on unoiled grates. Grill, uncovered, until lightly toasted, 2 to 3 minutes. Transfer bread to a plate; cover with aluminum foil to keep warm.

Place tomato slices on oiled grates. Grill, uncovered, until warmed and faint grill marks appear, about 30 seconds per side. Remove from grill.

Spread mayonnaise evenly onto the ungrilled sides of bread slices. Top 4 bread slices evenly (mayonnaise sides up) with bacon pieces, tomato slices, and lettuce. Sprinkle evenly with salt and pepper. Top with remaining 4 bread slices, mayonnaise sides down.