Is there an air/fuel ratio system that will bolt on a 496mag?

Any of the available systems will work as long as you have an O2 sensor bung welded on, or have an adapter plate made to fit between the manifold and the riser. I welded one into my riser. Not the easiest welding task for an amatuer like myself due to the fact that you have to put it in a double walled riser, which means welding on the inside of a tube. A good tig guy should have no problems.

I see dana marine has spacer plates for an o2 sensor....

http://www.danamarineproducts.com/Pr...TOKEN=78316251

Good find. Not sure these would work with a 496 exhaust though. Looks like they're made for Dana's Torque Flow exhaust. That's probably why no one makes spacer plates: Every exhaust is different.

Just built new EFI motors and had O2 bungs welded into CMI's for tuning. Find a local shop that can weld them in. The bungs are only about $10-15. (I'm not sure I would want them hooked up all the time though)

Yep, you absolutely don't want them in all the time. With the moisture in the exhaust on a boat, I wouldn't expect them to last very long. I only put one in when I'm using it.

I bought this one....2 channel....for tunning

I bought the FAST dual O2 system for a dyno, so it came with 22ft cables

That is incorrect. You DO want them all the time. If you have water issues they will fail instantly. With proper installation they will last a long time. The real time feed back of AFR can help diagnose running issues, i.e. whether it's a fuel problem or an electrical problem. I'm currently at almost 300 hours of full time use with wideband. I've found it so valuable I placed the AFR gauge where the fuel quantity used to take residence and move the fuel gauge to the gunwale.

To the OP, there are two parts to your answer. One is you need either a spacer to be placed between the lower manifold and riser/elbow OR a bung welded into your particular exhaust system. You then need one of several wide band kits on the market. I use the Innovate MTX-L. They have a new model and a dual channel as well.