Yes you can do that IF the WB 3.55 LSD is a large salisbury, not a small one. Just get a diff shop to do it for you.

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On the 2.60:1 thing, HX features manual dated 9/75 (updated 9/76) lists 2.6:1 as economy diff for 253 or 308 auto sedan, wagon, GTS or Statesman. Also listed as economy option for L31 M41 ute and va. 2.60:1 with 39:15 teeth listed on spec page dated 12/75.

I also have a hand marked up 1/74 version of the HJ features manual that has 2.78 crossed out all through it and replaced by 2.6 for a later revision. It appears at least in 4/75 the 2.78 was stilla round. There is also a revision of the production option availability document which shows G70 (2.6) added in 6/75 but there is a handwritten note date 5/76 staing next to "general revision" noting base axle for Statesman "was GU7". It also says the 2.7":1 was not available with XT4 (ADR27A) so I deduce from that 2.78 phased out near the end of HJ with 2.6 becoming standard on HJ Statesman a little earlier in 1976.