A new exhaust manifold means theres a good chance you will be torquing the brittle old manifold bolts off the cylinder head. Then instead of retapping one hole for your 02 sensor you get a chance to retap up to 8 on your cylinder head and maybe pulling the head so you can add a few helicoils too. Been there done that - no fun. I'd devote all my energy in getting the 02 sensor off, even if I had to drill it or use a torch.