No, an EGR Valve from a 97 will NOT fit in a 2000 Ford Mustang just because of the fact of the year and the part.
Look at the 97:
http://www.autopartswarehouse.com/details/QQFordQQMustangQQStandardQQEGR_Valve_Position_SensorQQ19961997QQSIVP13.html?apwcid=P1135867996W43b3f85c7ab9e&apwidS2ILwFR
Look at the 2000:
http://www.autopartswarehouse.com/details/QQFordQQMustangQQMotorcraftQQEGR_ValveQQ2000QQMICX2077.html?apwcid=P1135867996W43b3f85c7ab9e&apwidh2fu3Up
The era of Mustang evolves in 1999.
Look at the 97:
http://www.autopartswarehouse.com/details/QQFordQQMustangQQStandardQQEGR_Valve_Position_SensorQQ19961997QQSIVP13.html?apwcid=P1135867996W43b3f85c7ab9e&apwidS2ILwFR
Look at the 2000:
http://www.autopartswarehouse.com/details/QQFordQQMustangQQMotorcraftQQEGR_ValveQQ2000QQMICX2077.html?apwcid=P1135867996W43b3f85c7ab9e&apwidh2fu3Up
The era of Mustang evolves in 1999.