Can I put Thermocouples in my Stainless Marine manifolds? I know there is water jackets running through the manifolds, but would it be safe to put a thermocouple in the bottom of the casting about 2 inches in. I can't believe there would be water there.

Is 2" far enough away for EGT's. I don't know the distance it would be from the center of the piston. I was told #7 burns the hottest. Is that true?

I have done it on several brands. (mercruiser/revolution and Lightning) About 1" to 1 1/2" from flange. Drill slowly, if you hit a void then you are in water jacket. If that is case plug that hole and try again. You can feel with fingers where header tube starts to get thicker, this is where jacket starts.

Remember how far probe sticks out for clearance of starter/engine mounts etc. I think #2 would be ideal place for probe if you can make it work. Mine are in #7 & #8 due to above mentioned considerations.

If you look close at your stainless marine exhaust manifolds it is pretty easy to see how they were manufactured and where the water jacket is. It's a well thought-out cast manifold with an outer jacket welded to a manifold creating the water jacket. Look at the runners as they exit the cylinder. It's just solid aluminum until you hit the weld, that's where the jacket starts. Just put your probe somewhere between the cylinder head mounting surface and the weld and you should be in solid aluminum.
I hope this helps!
DS