I didn't pull any gear case vents; I put a wet rag over the gear case and welded an inch, then let it cool for 10 minutes, and welded another inch. In between these passes the rag didn't even start steaming, stayed cool as a cucumber. I'm sure it gets hotter sitting outside in the summer.
Aluminum does dissipate heat well but you also have to be careful with it, it's easy to get a runaway heat soak condition where the whole part goes from being cool to thermonuclear in what seems like a split second.
I clamped my ground right to the skeg, so no current to flow through any part of the electrical system. The electrical system on this thing is basic and all the ignition stuff is isolated from the battery anyway; it's all run off the stator to the ignition components. I have never had an issue with electrical systems and welding cars, as far as i'm concerned disconnecting a battery does nothing; if you're gonna fry a computer with high frequency start or magnetic fields, you'll do it with or without the battery.
I'm an electrical/embedded engineer so I have some vague sort of inkling of how electrical systems work...
.