When I did the conversion I didn't bother changing proportioning valves, and the brakes work fine, so if you do nothing it will be "OK".
I was talking to my mechanic about it and he said there is a spring which helps hold residual pressure, which is different for the drum / disc brakes. He said just pull the spring from the GC proportioning valve rear output line and use it to replace the spring in the XJ proportioning valve's rear output. I haven't done this yet since the brakes don't seem to drag using the stock XJ proportioning valve and I don't have a GC proportioning valve.
Not sure if this helps, but it may save you some hassle.
1998 XJ - built for offroading
2007 WK CRD - DashHawk, for towing the XJ