Hubcaps and wheel size

[Montierth]

I have a 70 GMC that is 5 lug. I have been looking at the trucks with 20 or 22 inch steelies and they look really good. My main option to go with that look would be detroit steel wheels. But i have seen some trucks that have what look like original hubcaps and then have 20 inch steelies. How are they attaching their hubcaps when they are meant for 15 in or 15.5 in wheels? Im interested in the full 15 inch caps and not the dog dish. Maybe im wrong and they are not putting them on 20 in wheels.