Can you put a AF wideband on a 496 mag . If so where does the sensor go and any reccomendations would be great . Does the closed cooling still exhaust the sea water through the exhaust manifolds . Does anybody make a sensor for marine application ?

Yes

Is this yes to the guage or to the sea water exit ?

THX were does the sensor go ?

Yes, sea water still exits out the exhaust.

You could drill a hole into the stainless riser about 5-6 inches from where the joint to the manifold is .
right thru both pipes ( double wall) the thread is M18x1,5 (wich means Metric 18 mm ,and 1,5 mm per thread rotation ) at least here in Europe they used that for 02 sensors., not sure in the US about the thread .
So the hole must be drilled 18 mm right thru both pipes ,now you need to drill the outer pipe something like 35 mm ,this makes clearence to the TIG torch to TIG weld the thread plug to the inner pipe ,when this is done ,you need to make a big washer that has a 18mm hole in the center and is about 40 mm so that is can be TIG welded to the thread plug and outer pipe .
This makes sure no water can,t escape and destroy the 02 sensor ( 02 sensors don,t like water )
Did this on several CMI,s and Stellings headers wich is no different than your stainless riser .