Do not ground pMags (or anything else) to the engine case. Run separate ground wires to whatever you are using for the common battery ground connection on the firewall. The engine should be connected to ground at only one place; the nice ground lug on the starter (#2 welding cable is perfect for this job).
Reason - if that starter ground wire fails for any reason the engine won’t crank. You will quickly find the problem. If you have a bunch of additional stuff grounded on the engine, the starter will now use these connections to seek a path to ground for all that current. Bad things will happen to all that stuff before you figure out why the engine won’t crank.
Carl
Sorry Carl - that is in direct conflict with the instructions provided by the manufacturer of the P-Mag. You are correct that if the ground strap fails, the current will find whatever path necessary to go back to ground - on a traditionally ignited engine, that would be the P-Leads. But for the P-Mags, a far better solution (if you’re worried about the ground strap failing) is to wire two ground straps. Going against the manufacturer’s instructions on an electronic ignition system is generally a bad idea.
You and I can disagree on this, but I want folks reading this thread in the future to understand that the manufacturer of the P-Mags would not agree with your recommendation, and they should do some research.