I researched that Opel GT yesterday and found that it was distributed by Buick in the US but it was widely available in Europe. That in itself kinda opens this up. It'd be cool if you could use an American car we're all familiar with but then again the whole idea behind this is to inspire all of us, artists and car builders alike, to think outside the box for the next build. I've seen some pretty sweet potential projects from across the sea, so I'm game.
Let's say this- with all due respect, (and I love Datsun 210's) lets' avoid Japanese products, even the cool old ones, as we're trying to design a car that will put the modern day Japanese imports that fill the streets to shame with classic American or maybe even European styling. Stick with American drivetrain products we're all familiar with. Also be sure it was available rear drive (for ease of chassis re-working) and had a 4 cyl as an option.
How about this too, lets all post a photo of what a stock version of the car you're doing looks like along with your rendering, so if it is a car we're not familiar with we can become so.