Peter Kenyon Reports On The Senate's Approval of revised Welfare Reform legislation yesterday. The latest version retains federal standards for medicaid eligibility and maintains food stamps as a federal safety net. At the same time it turns the main welfare program into a block grant with which states can design their own programs. It also cuts off many benefits after five years. President Clinton praised the changes, and the bill now heads for conference committee to be reconciled with a tougher House version.