Your WIP in post 15 looks good with the McLaren roof. The problem you have with the one in 16 is that the reflections on the McLaren don't fit that background. The McLaren looks like it was during the day with the reflections of the clouds and the reflections being so clear but the background is a dark foggy night.
I was having a play around with this earlier, I don't think you need to find a donor or complex rebrush unless you want to. The original eclipse paint fits the new bg alright with a few tweaks. A very quick mess around so very sloppy but to give an idea of what i mean, you could turn it in to something like this: https://c2.staticflickr.com/6/5543/22945887697_453c707820_o.jpg
Do a proper colour change and a few choice mods I think it would make a really good chop