The owners of Il Mulino, who, as we now know, are planning a new restaurant at 37 East 60th Street, have come up with a name and concept. Bistecca will be an Italian steakhouse with lots of leather, banquettes and a large bar. "At Il Mulino maybe we have one type of steak; here we will have 5 or 6,'' explained Jerry Katzoff, a partner in the restaurant. "The sides won't be typical of a steakhouse; there will be pastas and broccoli rabe.'' Of course there will be Il Mulino favorites, like bronzino and veal parmesan. "We expect to do a diverse lunch business, which we don't have downtown,'' said Katzoff. Though he maintained a deal has not been finalized, Katzoff is hoping for a June or July opening.