Using a sharp knife, cut a 1-inch crosshatch pattern in fat cap of roast, being careful not to cut into meat. Rub salt and pepper over entire roast and into crosshatch. Transfer to large plate or wire rack set in a rimmed baking and refrigerate, uncovered, for at least 24 hours or up to 4 days.

Open bottom vent of charcoal grill completely. Arrange 40 charcoal briquettes, 2 briquettes wide, around half of the grill's perimeter, overlapping slightly so briquettes are touching, to form a C shape. Place a top single layer of 20 briquettes, 1 briquette wide, centered on top of first.

Place 2 wood chunks on top of charcoal about 2 inches from each end of C. Place remaining wood chunk in the center of charcoal C. Place disposable pan in center of grill, nestled in between the coals, running lengthwise into arc of C. Pour 6 cups water into the disposable pan.

Light a chimney starter filled ¼ way full with charcoal (about 1 ½ quarts briquettes; pile briquettes on 1 side of chimney so they catch). When coals are partially covered with ash, use tongs and pour to place them at 1 end of C, overlapping just the end of the coal arc so the hot coals will ignite the unlit coals.

Set cooking grate in place, then clean and oil grate. Close grill with lid vent oven completely and let heat up until hot, about 5 minutes. Place roast on oiled cooking grate, centered over the water pan that is below the cooking grate, fat side up, with rib bones facing the charcoal arc. Insert a temperature probe (if using) into the center of the roast. Close grill with the lid vent completely open, positioning the lid vent over the roast. Cook until meat registers 125°F (50°C) for medium, 2 ½ to 3 ¼ hours.

Transfer roast to a carving board, tent loosely with foil, and let rest for 45 minutes. Using a sharp knife, first carve the entire loin of meat from the bones in one large piece. Next, slice the roast ½ to ¾ inch thick. Serve with horseradish sauce. (Leftover meat can be refrigerated for up to 2 days.)