Sam Pack’s assemblage of automobiles could fill a municipal garage. But even Dallas collectors have to downsize sometime.

A light mist falls from low gray clouds, but inside Sam Pack’s private car museum in Dallas, it is as bright as if the sun blazed above. Mustangs, Bel Airs, Thunderbirds, Cobras, and dozens more shine under the overhead lights. A man feather-dusts them, though there is not a speck of dirt in the place.

Pack, a Dallas-area Ford dealer, has a staggering personal collection of cars: 500 …