The only place I know of to mount a sensor in stock cast iron exhaust is the bottom side of the manifold near the exhaust port. If you take a stock manifold and turn it over you will see how the paint is burned about a inch or 2 back from the exhaust port,the manifold is single wall at this point,you drill and tap the manifold for your 02 sensor at this point. Put a sensor in #7 and #8,tune for whichever one reads the leanest,don't get to crazy trying to tune it too exact as your not getting a true average of all the cylinders. This is how Tyler Crockett had me mount mine in my stainless marine 3's and right or wrong it has worked for tuning,Smitty