Peter,
You could also try Jonathan Bradburn. He has several gear boxes on his website and also advertises on ebay where there is a CR7 box. Also Brian May might be able to help (tel. +44 121 4303767). Best to phone him and he is very helpful. Good luck in your search.
You are right Geoff Dowdle's 400 does look fantastic!! Has it given you ideas?
Best wishes from the far north
Richard