How much will it cost? If you have to ask you can't afford it
Seriously, be prepared to spend big. I know the sort of money that John spent on his Nationals-winning convertible FE, and it's not for the faint-hearted.
Basically you have to fabricate all the parts (stretched doors, stainless trim and so on) yourself.
Your first step (if you're in Victoria) is to go along to their club meeting (Sept 10) and have a talk to John - he's a very approachable guy.
You will also find a copies of magazine features on John's cars here:
http://scad.com.au/jwswing/jwholden/index.html, including the Street Machine one. That'll give you some idea of the scope of the project you're considering.
cheers
RET