When I did mine I removed the big nut and pulled the Piston out partly because I wanted to inspect the Piston and the Spring (ounce in a while the springs break). I also did it becaues it is easier to get the O-ring in if you can also push from the Piston side with the Piston rod.
If the Pump is already off I would put the Pump in a Vice to remove the 30mm nut (if you chose to do so).
Also while I only took it apart to change the O-ring I took the whole Pump apart to take a look. I had no rebuld kit so I too some 400grit emery paper and sanded the face of the plastic valves to be sure they were flat.

If you put the Pump back on the IP to remove the 30mm Nut I doubt if it would break the studs if the Stud Nuts were tight. Otherwise I think it could twist the Studs.

The 30mm Socket I have is actually a Deep Socket and when I tried to get that Nut off while the Lift Pump was on the IP I felt I could not get into a good position to loosen and I knew using the Vice would work so I just did it in the Vice.

Also I had no rebuild kit so I heated the Copper Crush Washers Red hot. This according to my old Bosses Navy Machinist Manual softens them so that the can be reused. It does not matter how they are cooled as long as they get Red hot.

Note: Do not do this heat treating with Delivery Valve Crush Washers unless there is no other choice.

I took this pic for those who might want to remove that 30mm nut to one of the safe positons it can go in on the vice. However, without some Copper or Aluminum Jaw Pads this Pump wants to slip out of the Vice.
I could not find my Aluminum Pads when I took the pic.
The one I put the O-ring in is on the Car. In the pic is the Pump from my spare IP.

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84 300D, 82 Volvo 244Gl Diesel