With almost 100,000 miles, my Challenger didn't show any signs of ovaling out the holes in the frame. I've heard of that happening when people install through bolts instead of the factory setup. Also, you can crush the frame with a through bolt.
I think the best thing to do if you don't care about looking original, is to open up the holes and weld in some spacers.(like you find on the alternator/power steering brackets) That way you can't crush the frame and you have more meat there.