A sensor for ECU control does normally have a dedicated place on the block, but I've put them almost everywhere in the past to listen to knock. Even on the metal back plate which bolts the engine to the dyno, so I wouldn't worry too much. Remember it's only the high frequency stuff that you need to pick up

If you used an inlet manifold stud I don't think you'd get the full surface area of the sensor touching metal would you?

Are you using the standard ECU or aftermarket?