Factory system uses clamps. Manifold back implies a new catalytic converter, but a cat should live through two to four mufflers and tailpipes, so why would you want to weld the muffler to the cat?
If you get a direct fit cat, it has a flange at the forward end to mate up with the exhaust pipe (down pipe). If you go with a universal fit cat, you might weld the cat to the exhaust pipe, but I would suggest using clamps from the cat back.