Yes you can, though it's relatively expensive. "SPEED (ltasrusa)" at the Ace Cafe on Delphi Forums employed a 18" aluiminum rim from Sun Rims (very high quality stuf, I think) and I believe, an 18" Metzler tire.
http://forums.delphiforums.com/acecafe/start.
Log into the Ace cafe, pull up any of Speed's posts and email him fom there. He's VERY sharp, figures out how to do stuff while everyone else seems to just be talking about it. He also detailed some of his process on Delphi's New Bonneville Forum, post # 5549.6. Bill H