Okay... so this was a total failure haha. I got the dc-dc converter installed tonight. However, I really just didn't think this through. I cut the wires right in front of the ignition coil... but those don't carry a constant 12V. The ECU fires the ignition signal to them as needed. Well, the dc-dc converter just won't work under those conditions. It needs constant 12V. The engine wouldn't even fire up with the dc-dc in the circuit. Looks like I'll have to boost the voltage to the ECU if I want to do this. That probably means I'll need a higher rated dc-dc to fully supply the power the ECU needs. I'll look into that.
So, it sits like this with some electrical tape around it. I'll solder it back up and get some shrink warp on it later on.
However, I still have this dc-dc converter and I have another just like it laying around. They're rated for 80W, and I think they'd probably do a good job running power to the headlights. So, I think I'll use them there instead.