[Gardener]

When I have similar issues, I don't do levels, I just put all the projects in parallel, maybe with a brief indicator of the very top level. If a project is dependent on another, I give it a WAITING FOR next action that refers to that project. In the weekly review, if the "waiting for" project is done, that's when I take the project off hold and give it a Next Action.

So, using your examples, the following would all be at the same level:

Project: 292 Motor (Chevy)
Project: Front axle (Chevy)
Project: Transmission tab issue (Chevy)
Project: Transmission (Chevy)
----ON HOLD WAITING FOR: Transmission Tab Issue
Project: Research brake lines (Chevy)
Project: Brakes (Chevy)
----ON HOLD WAITING FOR: Research brake lines

So, everything that can be productively worked on its own track, gets its own project. There could be three or four more projects that Transmission or Brakes are waiting for. When those lines of activity converge again, that's when I'd return to the original Transmission or Brakes projects.

The ON HOLD WAITING FOR stuff does add complexity, perhaps as much complexity as nested folders would. But it's complexity that I deal with at weekly review time, when I'm geared up for complexity, instead of whenever I'm looking at my action lists, when I'm not. When I'm looking at my action lists, those on-hold projects don't even need to be visible, and the active projects aren't buried in folders.