You really only need that tool if you are going to pull the valve part of the clutch pack apart and replace the main o-rings on each. The kit you purchased from the US should have everything you need including the countersunk screws in the rear of the box. Mine were so tight l had to drill most of the heads off to get the innards out!!
Easy box to do a clutch pack and steels kit rebuild on but a lot of parts. Be methodical and careful. Lay the box out in order as you pull it apart, clean clean and reclean. Inspect and if in doubt, replace. No fluffy or stringy rags, and plenty of quality trans fluid when it goes back together. Soak the clutch packs before assembly.
There is a Jag auto overhaul manual on the interweb that gives you the strip down and replacement procedure, it's the only one l found that gave you everything you needed. Best of luck. My box has done some 10000 km since open heart surgery and a C1 one way sprag replacement, so if l can do it anyone can.

