Vince -
The Durango units are spec'd for 136A max output, and are (I believe) the same size case as the OEM ND units used in the XJ.
Bear in mind that, if you want to get full output from the thing, you're going to end up having to replace the alternator output fusing (either fusible link or two MAXI60 fuses in the PDC,) since full output from the Durango will likely pop the fuses otherwise.
Apart from that, everything I've read on the subject says it's a fairly straightforward swap - no fabrication, but you may need to adjust a thing or two (you'll see when you get there.)
5-90