Does anyone have the part number for a Gatorback belt for a 1998 4.2 with A/C? The guy at the parts store said there are several different sizes for this engine.

 

There are 2 different sizes:
103 inches: 4061030
89.5 inches: 4060895

Look on your stock belt and you should see some numbers printed on the belt.
The number will have 1030 in it or 895. That's the size you will need.