Weld a nickel or somthing that size to block two of them, and then the one that makes sense to have the O2 sensor go to ,where stock placement is, weld a stock bung in there. You can put a wrench or socket on an s14 or s13 stock exhaust mani O2 bung and simply unscrew it, once you have it out drill out the bung in your new header to fit the stock bung,weld it in place and voila