The U.S. Environmental Protection Agency is expected to release this week what clean air activists call the most important strengthening ofautomobile tailpipe emission rules in nearly a decade, administration officials said yesterday. Once in place, the rules would have the effect of taking 54 million cars and the pollution they produce off the nation's highways, according to the Clinton administration sources. It also marks the first wide-scale change in the nation's emission rules since the 1990 Clean Air Act.