ByJohn S. McClenahen In what may prove to be one of the understatements of 2003, Maury Harris, chief U.S. economist at UBS Warburg LLC, predicts that Capitol Hill lobbying against President Bush's economic stimulus proposal "will be fierce and some changes are inevitable." Specifically, Harris anticipates less tax relief for upper-income households and some reduction in the overall size of the $670 billion proposal. The White House is eager to have Congressional approval for the package by April 15, the date that initial estimated income tax payments for 2003 are due, notes Harris. But passage that quickly could be difficult to achieve. To assure that the legislation can make it through the U.S. Senate with 51 votes, the size of the Republican majority in the chamber, and not be delayed by a filibuster, the tax cut must be attached to a budget reconciliation bill, Harris explains. And there's no guarantee that will happen.