WASHINGTON, March 6 — With Congress heading into a politically perilous budget season, influential House conservatives plan this week to propose an austere alternative spending plan that would pare more than $650 billion over five years, balance the budget and drastically shrink three cabinet agencies. The legislation, part of a push by some Republicans to re-establish themselves as champions of fiscal restraint, was taking shape as President Bush struck a similar theme on Monday by asking Congress to grant him line-item veto power to eliminate federal spending that he might judge wasteful. "We can't be all things to all people...