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I've recently learned that you aren't supposed to ground the ECM case.
Although I'm running two 7747's in different rigs that are grounded and they work fine.
If you don't have A/C, the passenger side FW has two bolts sticking out that hold the heater core box on. Those are excellent places to mount the ECM (if you disregard the fact that they aren't supposed to be grounded...).