The firing order is 1-5-3-6-2-4
Number 1 is the first post clockwise past the forward mounting screw of the distributor.
Timing on 4.0 XJ's is not adjustable, handled by the engine computer.
Because there is no provision for distributor rotation to adjust timing, if the engine doesn't start and run properly, you may have to experiment with reinserting the distributor to get it indexed properly. The rotor should point to terminal one of the distributor when the engine is at TDC, and since the drive gear is helical, you will have to start it with an offset to compensate for the rotation as you push it down.
Start by rotating the oil pump slot to a little bit before the 11 o'clock position, as seen from the passenger side (12 o'clock is pointing toward the engine). Later distributors have an index pin that can be inserted to force correct installation, but I don't think it's present on yours. But the goal is to get the rotor pointing to the 5 o'clock position when it's in.