The story is nearly right. The car was actually shipped CKD and assembled by Mazda in Japan which is why they have numerous unique features that we never saw in the original Holden Premier. It is also a shorter wheelbase and wider track.

The tie-up was not due to the Rotary engine, but the fact that Mazda Japan assisted in GM and Isuzu coming together in 1971. Isuzu sold the Holden Statesman in Japan under the Isuzu badge before the Mazda Roadpacer came into existance.

Still, a good article on nearly the rarest of all Mazdas. Only the Rotary Parkway bus and original Cosmo L10A sold in fewer numbers.

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Mazda T600, R360, Carol 600, Familia 800 and 1000.
1960s Mazda piston power!
1989 HC10E 929 & Luce Turbo hardtop
1977 Honda Z50 J1 Mini Trail
Then there are the three, sorry now four, fire engines.......

Can you explain how & why this would be done?

Curious as to the mechanical changes & what the benefits would be

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Because the Rotary engine sits higher in the engine bay than the original Holden V8, they needed to use a two piece tail shaft (Holden originally was a single piece) to meet the correct angle, but that required the rear axle to be moved back slightly to make it all work. I originally thought forwards but have checked the specs online and the Roadpacer is longer not shorter. Holden is 2819mm wheelbase and the Roadpacer 2830mm.

The specs online do not appear as accurate as I remember as they state the same track for both at 1530mm. I have a brochure at home that I will check the track variation with but maybe I am wrong on the track? I will check and confirm.

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Mazda T600, R360, Carol 600, Familia 800 and 1000.
1960s Mazda piston power!
1989 HC10E 929 & Luce Turbo hardtop
1977 Honda Z50 J1 Mini Trail
Then there are the three, sorry now four, fire engines.......

Interesting:

http://en.wikipedia.org/wiki/Mazda_Roadpacer