Its because Marcus has two conversions on his website.
An EA and a BA conversion.
The BA does not show dimensions of the spacer.
The EA conversion does. The spacer is 10mm.
http://gojeep.willyshotrod.com/Brake%20spacer.jpg
http://gojeep.willyshotrod.com/HowtoRearDiscs1.htm
Not sure on the differences between the two setups.
For my rear disc conversion, i used the ZJ setup. It was cheap and easy to do and ive had perfect rear brakes ever since. No spacers and it also meant i could keep my ABS intact, so a win win situation.
The write up for my conversion is on my VSB14 thread in the XJ section.
Cheers,