1. Pat roast dry with paper towels. If necessary, trim thick spots of surface fat layer to about ¼-inch thickness. Using sharp knife, cut slits in surface fat layer, spaced 1 inch apart, in crosshatch pattern, being careful not to cut into meat. Sprinkle roast evenly with salt. Wrap with plastic wrap and refrigerate for at least 6 hours, up to 24 hours.
2. One to 2 hours before grilling, submerge wood chunk in bowl of water to soak.
3. Open bottom grill vents fully. Light large chimney starter filled with charcoal (6 quarts, about 100 briquettes) and allow to burn until coals are fully ignited and partially covered with thin layer of ash, about 20 minutes. Empty coals into grill to cover one-third of grill with coals steeply banked against side of grill. Place wood chunk on top of coals, position cooking grate, cover, and heat until grate is hot and wood is smoking, about 5 minutes. Scrape grate clean with grill brush.
4. Sprinkle roast evenly with pepper. Place roast on grate with meat near, but not over, coals and bones facing away from coals. Open top vents halfway and cover grill, positioning vents over meat. (Initial grill temperature should be about 425 degrees.) Grill roast until instant-read thermometer inserted into thickest part of meat registers 140 degrees, 1¼ to 1½ hours.
5. Transfer roast to carving board, tent loosely with foil, and let rest for 30 minutes.