I have fitted power steering to many HQ - Zs without changing the arms, and really didn't have any problems. From memory a lot of HQs had short arms with manual steering. And as the models continued, more manual steering vehicles were fitted with the long arms. But that could have been mostly utes & tonners that had the short arms...(once again from long time fried memory!).

Also most of the vehicles I converted years ago were extremely lowered and accuracy wasn't a priority.

I would have a chat to a front end specialist & let them give a pro & con on the steering arms

cheers
Max