Sumner Redstone is defiantly pushing ahead with his Blockbuster IPO. Despite weaker-than-expected first quarter cash flow from the 6,400-store video rental chain, Redstone's Viacom Inc. plans to register for an initial public offering of Blockbuster stock in May. The IPO is likely within a few months of the filing. First-quarter cash flow at Blockbuster, to be announced in a few weeks, is expected to be down by as much as 15 percent because of increased spending on promotion. As recently as early March, some analysts were expecting 24 percent growth at Blockbuster. But company execs said they decided to continue an expensive marketing campaign that had been more successful than expected in building market share for the business. Revenues at Blockbuster are expected to rocket some 20 percent in the first quarter, on some 15 percent same-store rental unit increases, Wolzien has reported. Viacom chief Sumner Redstone said recently that he expects Blockbuster to start showing growth in the second quarter.