You want the 275/50/15s on the 15x8s. When I first was about to put the drag radials on my 15x8s, it took me almost FOREVER to decide, but I went with the 275s. The right choice. You'll love them.
Oh, and here's how they'll look (these are Nittos, though):
BTW, my rear fenders lips are rolled-- no big deal or hard task. You may need to do this so they won't scrap, but since you're getting 50s, tehy may be just fine. I rolled mine b/c I previously had some Eagle tires that were 255/60/15s and they did scrape the fender lips when I hit bumps on the road.