71 valiant paint code question

In 1971, the color GW3 was called "Sno White". Take a color sample to your local paint supplier. If you know it was originally GW3, and they have a sample of what it looks like now, they should be able to mix something really close. Don't try and match it out of a pre-mixed spray can. If you really want it in an aerosol can, most paint supply shops can do that for you too.