Hi Dai, while I'm a huge fan of the Alfa V6 and would love to see you put one in your Dehavilland it does make sense to consider the KV6 from a Rover/MG/Kia. Theres a page on the MGF website where data about such a swap is gathered, including a schematic, overlaying the KV6 on the K4 fitted to an MGF:
http://www.mgf.ultimatemg.com/group2...ne_options.htm
If you google the GTM libra fitted with the KV6 you'll find plenty of photos, videos, build diaries that may be of interest too.
Incidentally it would be valuable for everyone here trying to plan a V6 transplant if you could take some photos and measurements of your car. What is the height from the ground to the engine cover in a Dehavilland, for example? What is the track and what is the wheelbase (is it 2280mm like a Dino 206GT or 2340 like a 246GT?)
Best wishes for your project.
Ross