I just want to make sure I'm not breaking any rules/ethical boundaries:
I've downloaded all these assets for vehicles to use for a map which features sky traffic, is it okay to turn them into props but still credit the authors, or do I need the explicit permission of the authors to use their vehicles as props?

You're being waaaaay too cautious. Unless the author explicitly states in his or her readme that the asset must be used as a vehicle, which nobody has ever had, I don't see what the problem is making it a prop. There are worse things one can do with someone else's assets, like claiming authorship.

Okay, thanks, the thought just occurred to me, and I figured it'd be smart to play it safe.