Number two, given that you had a leak develop right after the overheat, that seems to suggest pretty strongly that the head warped. :o/
That said, the sputtering until you disable the EGR suggests that the secondary plugs may not be doing their job. Looking at the wiring schematic in my FSM, there is a fuse that handles _only the exhaust side plugs_. That's the first thing to check. datzenmike has the layout of the fuse box here:
So, I'm trying to make sense of my wiring diagram. First off, it doesn't seem to indicate wire gauges, so I dunno if the signal wires are supposed to have the same gauges on the two sides. My guess is that they should, and so that lighter-gauge wire could also be causing a voltage drop to the input of the exhaust coil, but that's just a guess. First things first, hit up that fuse.
Here's a wiring diagram. It's not as useful as the one in the '86 FSM (EL-37 and EL-38 are the ticket), but it's online:
http://nissannut.com/projects/z24i_fuel ... wiring.jpg
Note that this is my first time reading an auto wiring diagram, but I'm pretty familiar with electronics from other contexts. That said, the wire colors are kind of confusing:
1) Brown is the source of power to the intake coil. At some point, Brown is connected to White/Blue, which connects to the ignition and to the top side of the ignition section of the fuse block (aka fuses #1-#5).
2) There is a White/Blue wire on the trigger side of the intake coil. _This is a different wire than White/Blue from #1_. This wire goes to the distributor for the trigger signal on the intake side
3) Black/White is the power source to the exhaust coil. This connects to the bottom side of Fuse #1 (10A). As such, it should be at the same voltage as Brown (aka wire #1), assuming that wiring is all in good shape, and the fuse is in good shape.
4) Red is the trigger signal for the exhaust coil. It connects directly to the distributor with no detours.
Note that there are some other connections. Wire indices match above:
1) Brown also connects to "Condenser", which is basically a cap. Presumably, it helps to compensate for voltage drops due to the coils charging.
2) White/Blue also connects to a resistor, the other side of which goes to the Tach as the tach input signal
3) Black/White connects to "Condenser" as well. It has a distinct cap from #1, although both caps share a ground.
5) The shared ground from "Condenser" is a Black wire that connects directly to the grounded body of the distributor. The schematic shows it going through a "Plain connector."
6) There is a Brown wire on the bottom side of Fuse #2 (15A). This is _not_ related to the Brown wire in #1. The power supply page (EL-5) shows that this wire powers "Inhibitor switch, Engine control system (Auto choke heater, Fuel cut solenoid, Detonation control unit)
So, just to reiterate:
— Brown from intake coil hot side is the same wire as White/Blue at the top side of the Fuse Block.
— White/Blue on the ground side of the intake coil is the trigger signal, and serves as the tach input signal as well
— Brown from the bottom side of Fuse #2 is different from Brown on the power side of the intake coil, but they are actually connected through Fuse #2