I spoke with MSD tech and was told that it can be mounted upside down or anyway for that matter as long no liquid can get inside. And to keep the coil wires away from any item that may be affected by the high outputs (i.e. speaker wires etc.) and although the instructions says that you can ground the unit to the block, the tech was very specific about connecting the ground wire to the "-" post of the battery, which I intended to do anyway but just an FYI.
Yeah, I've had mine mounted under my rear seat delete for over a year without any problems. Plus it stays nice and cool since it is out of the engine bay.