What you need is a spark plug antifouler. 18mm. You can get them at AutoZone, PepBoys. Most parts stores. 2 come in a package. You need 2 so just one package. They are about $3. You take one of them and drill it out with a 1/2 drill bit. The other one you leave alone. The one you drilled out your O2 sensor screws into it, and then it screws into the other one (the one you did not drill). now you screw the whole thing into your hole for the O2 sensor. Make sure you tighten it really good. You are done.
You only bore out the end of the fitting where the sensor needs to fit threw. You should have clearence and be able to leave the threads intack to screw in the new O2 sensor