Hi,
I asked the same question just over a year ago and got a pretty detailed response. I particularly wanted to go down that "All Holden" route and did not want to look at any other available boxes that may be far better. There are adapters from Dellow and Castlemaine Rod Shop (CRS) available, or were available some time back that enable a wide range of Japanes boxes to be fitted up, without requiring the transmission tunnel surgery to fit an Aussie 4 speed.
http://fefcholden.org.au/forum/index.php/topic,12876.0.htmlI don't know if you wish to retain the original seat, if so then the following article may be of use to you. If you do, send Alex (Custom FC) a PM and he may be able to restore the photo's to the thread.
http://fefcholden.org.au/forum/index.php/topic,750.0.htmlRegards
Keith