Preheat oven to 350°.
Trim most of the visible fat cap from top of roast. A thin patch here and there will help to keep roast moist during cooking. Reserve a strip to place over the seam that will be on top of stuffed roast.
With a sharp knife cut down into the roast, lengthwise, about half way. Turn the knife to the one side and cut horizontally stopping about 1 12" from the edge. Repeat to slice the other side of the roast. You should be able to 'open' the two side flaps and have roast almost lay flat.
Lay 3 kitchen twine strings horizontally on your work surface. Position the butterflied roast so the twine is evenly distributed with one an inch or so from each end and one in the middle.
Take stuffing and form a log lengthwise in center of the roast. Pack the stuffing and use as much as you can but top seam should meat. Transfer any excess to a separate casserole to heat separately in the oven.
Close up the roast. Pull the twine as tightly as you can and knot it to hold the roast together. A second set of hands is helpful here. You can also use turkey skewers on the seam to help keep the roast tightly close.
Season the roast on all sides with salt and pepper.
Lay a strip of the reserved fat cap along the seam to help keep the edges moist.
Oil the bottom of a baking dish or line with a wire rack.
Place roast in baking dish and cook about 1 ½ hours until internal temperature reaches 145 degrees. Cover casserole of excess stuffing an put in oven 20 minutes before roast will be done. When done remove roast from the oven and tent loosely with foil for 10 minutes. Ensure extra dressing is well heated through.
Cut away twine from roast. Slice 1-2" thick and serve with Port Cranberry Sauce.