Best place for ground under dash?

With EFI, the engine is a better ground reference than engine than the battery. Why? Because when cranking, the large current and associated voltage offsets the engine sensor that are ground referenced. Sensors like temperature, and O2. So the measurements are immune to the cable drop. It also protects against the ECU and associated wiring for loss of engine ground. Other wise the ECU wiring fails to deliver the starting load, it is not rated for.