Place the salt blocks on a grill grate of a gas grill over low heat, close the lid, and warm for 10 minutes while you prepare the chicken. Turn the heat to medium and heat the block for 10 more minutes. Its surface should be about 375°F. If you are using a charcoal fire, set up a bilevel fire with half the grill set up for low heat (one layer of coals) and the other half set up for medium heat (two layers of coals). Remove and discard the neck and package of innards from the cavity of the chicken. Place the chicken, breast side down, on a cutting board. With a large knife or poultry shears, cut down the length of the spine on both sides. Remove the spine. Cut the breast side of the chicken in half lengthwise. You will now have two chicken halves. Wash the halves in cold water and pat dry with paper towels. Coat with the olive oil and rub all over with the cut sides of the garlic cloves; afterward tuck the pieces of garlic under the edges of the skin. Season all over with the pepper. Clean the area of the grill grate not occupied by the salt block with a wire brush. Put the chicken halves, skin side down, on the grill grate and, using grill gloves or thick oven mitts, put a hot salt block on top of each half. Close the lid and cook until the chicken skin is crisp and deeply grillmarked, about 15 minutes.Remove the blocks using the grill gloves, flip the chicken halves with tongs, put the blocks back on top of the chicken, close the lid, and cook until an instant-read thermometer inserted into the inside of the thicker thigh registers 170°F, 10 to 15 minutes. Remove the salt blocks, transfer the chicken to a clean cutting board, and let rest for 5 minutes before cutting into parts. add lemon juice and serve.