Hello Iain,
As Ilian said, i just finished an Italeri/Airfix kitbash on this vehicle. The conversion is easy enough and measures out almost perfectly to 1/72. I´m sure the Roden 4x2 Blitz could be used instead of the Italeri one. I just used what i had available at the time.
Here are some links to my build:
http://www.network54.com/Forum/47210/message/1330369258/Opel+Blitz+Allrad
http://www.network54.com/Forum/47210/message/1331052683/Allrad+Blitz+ambulance
http://www.network54.com/Forum/47210/message/1334114026/Opel+Blitz+finished
/Peter