If the block is properly grounded, then JRMiii is right, there should be no real difference. The 'spark' is headed to ground across the gap anyway, this is just getting it there through a different path. Electrons flow, they don't care how they get there. If you're trying to get technical, the only issue I could see is with the return currents. Electrons are funny little things, and they try their hardest to travel in a loop of least resistance, but if given an option, they also travel along the parallel path they were sent in. This is kind of a funky law, and it usually only applies to PCB boards, but depending on how your wiring harness is set up, there are many ground wires and it may not tend to travel along the one you'd think. The whole point of this is to say that if the return currents choose to travel from the block (headed back to the battery) and through the same ground lines as the ECU, then you'll get big ground bouncing in the ECU, and if it's big enough, it could cause it to "see" bigger voltages than it could handle. Off the top of my head, I'd say the odds of that are low, but that's the only thing I could think of that would blow the ECU.