There's no way 20s will go without major body mods. 17's with a really low profile tire are the largest I've seen under stock fenders. Just remember that looks are one thing, but ride quality suffers and turning fender rubs are really irritating if you actual plan on driving your car, with oversize wheels. And don't plan on parking it anywhere without worrying about gangs stealing them FAST!
Wheels that big would look ridiculous on a classic anyway.
