Once a cowboy, always a cow boy. Robert Mitchum may have died at 79 on July 1, but TV viewers nationwide can still hear the legendary tough guy touting the goodness of red meat in commercials for the American Beef Council (”Beef. It’s what’s for dinner”). And that’s still his voice on ads for the Oppenheimer Fund, too. ”Let’s put it this way,” says C.J. Valenziano of the National Cattlemen’s Beef Association, which cosponsors the meat ads, ”he didn’t die a tragic death, and he does have an enormous appeal for consumers. We just felt it was okay to keep using him.” But only for a little while. Both advertisers are launching new campaigns this month. The good news: Mitchum’s rumbling voice will continue pitching Pontiac minivans.