To thaw a whole turkey in the refrigerator, you allow 1 day for every four or five pounds, so we’d set aside two days for such a large turkey breast. It may not take the whole time, but it’s better to have it thoroughly thawed than icy in the middle. Let the breast come to room temperature just before cooking it.
We would rub some butter or oil on the outside to keep it moist. You would do well to baste the meat every 20 minutes or so, and may need to put a piece of foil over the top after the first hour and a half to keep it from drying out.
In a 325°F oven (160°C), an 8-pound breast will take about 2-1/2 hours to cook through. According to the US Department of Agriculture, the breast should reach an internal temperature of 165°F (74°C), although many cooks would find that slightly overdone. If you take such a large piece of meat out of the oven 5 or 10 degrees before the desired temperature is reached and let it rest for 15 minutes, it will continue to cook and come up to the desired temperature.
With regard to stuffing, are you intending to make a pocket in the breast and put stuffing inside it? The more common approach would be to mound the stuffing under the turkey in the roasting pan or cook it in a separate dish. Stuffing is one of the foods that seems most susceptible to the development of harmful bacteria, so you really should use an instant-read thermometer to know that it is cooked thoroughly. The center of the stuffing should reach a temperature of 165°F (74°C). If you stuff the breast itself, by the time the stuffing reaches that temperature, the meat will be seriously overcooked.