The day before: Place your roast on a large baking sheet.
With a small pairing knife gently stab the meat several times all around the entire roast making slits. Not viciously but just enough to add slits allowing the spice and BBQ sauce to permeate the meat while it marinates.
Sprinkle on garlic powder and spice mix rubbing it all over the meat, even in the slits you made.
Place meat in larger than normal plastic bag. Pour in the BBQ sauce moving the roast around to ensure it is entirely coated. Seal up bag and place in refrigerator for a minimum of 12 hours.
The day of the BBQ: Remove beef from bag one hour before cooking and place in large roasting pan. Let sit on counter until ready to BBQ. Put your wood chips in a large bowl of water and let them soak for 30 minutes up to 60 minutes.
Get your smoke box ready by filling with wet wood chips. It's OK if you do not have a smoke box, simply place wood chips on a large piece of tin foil, loosely close it up and poke some holes to allow smoke to escape.
Preheat your BBQ to around 250F. Once ready keep your burner on at one end of your grill, and leave the other side off. Arrange the smoke box over the flame and close lid to let it start smoking. Once you see smoke place your roasting pan with roast on the burner that is turned off. Add 1/2 cup water to pan and close lid.
Check the wood chips and your roast every 30 - 60 minutes and replenish as needed. This cooking method is low and slow and will take approximately 4-6 hours depending on the size of your roast. Add more water to keep some liquid at the bottom of the roasting pan and baste your roast with the liquid each time you check it.
The meat is done when the inside temperature reaches ~160 degrees. When you check the wood. Remove from grill, tent with tin foil, and let sit. Once cooled we use a meat slicer to slice into very thin pieces, placing back into the roasting pan with juice, some additional warmed beef broth and more BBQ sauce. Cutting and shaving it thin with a sharp knife also works.
Mix together and serve with buns.