Duncodin, as yours is going to be as close to the real 507 as you can make a z3, check out the build of a guy that I've been following in ferrarichat.com under the title my BMW 507 project.
You'll need to set up a login so that you can see all the pics. He's in the US and is the guy who sent me scaled drawings of the side vents....I adjusted to suit a smaller area and took the taper out as a personal choice.
It's an amazing thread.
It shows how he built the car from scratch and theres a sectio on how he recreated his rudge wheels. Real nice guy that I've interacted with on a many occasions. His car would be number 253
Started by making a buck. His name is Dave and with a guy with such a skill set, you'd think he'd look down on our fibreglass "nod to's"....but he doesn't, because he's aware of the work that goes into any build.
He recently uploaded a drive by video...stunning. I sent him pics of mine and he said he'd be proud to be see driving it. As I say, real nice guy.
Check him out.