Engineers had to adopt over to the new VCSS which is a nightmare for mods.
Where as the old system was a "yeah, she looks ok maybe some brakes mate" approach.
You were probably better off running a HJ as the engineers are working strictly to the factory configuration and modifications based off it. Which as you say is where your dilema sits.
Emission, braking chassis etc are all ok and should pass on their own merit.
Under VCSS the vehicle has to pass engineers to get the rego and you'll have the find the right one to sign off.
Try
http://oldholden.com and start there. It's enthusiast site for everything pre VB. Someone on there will be able to steer you straight on how to get it through.
PS: It's a great site and some really awesome build threads in there...