you can only use the 3.8 flex plate from the 99 (which is externally balanced) if the 4.2 is also externally balanced otherwise if its balanced internally, you need a neutral flexplate
2003 3.8L V6 4R70W
Windstar Upper Intake with cool-air intake, MAC LT headers, BBK O/R H-pipe, and flow 40s, SCT tuner, FRPP Alum. D/S, 3.73 gears, Auburn HP limited slip, and Team Z LCAs, Eibach sway bars.
Best 1/4 time: 15.036 @90.92mph, 2.148 60'