I'm 99% sure the 4.0's after '95 used metric rings. However, the bore on the 4.0's is the same, so as long as you're replacing all 6 pistons, you can simply use the proper rings for the pistons. Where are you getting the parts from? I highly recommend Falcon Global Parts and Supplies (<--google that and call their #). When I bought parts for my stroker I used the 802CPs and they were $16.55 each from Falcon!
The ring kit I used was by Perfect Circle, pt# 51870CP and it was $41.
2000 WJ 4.5L - Stroked, Locked, & Long-Armed
Looking for 6" WJ springs!