We use da Vinci cranks for the same reason - custom lengths. Ours on the other side we like 160mm stoker cranks. We also like the option of using 110mm rings instead of 130mm rings. This allows a wider variety of custom gearing. I also like the shinny silver look. I just wish they made their sync rings in silver too.

I suggest purchasing a SKS square taper bottom bracket. 80 grams heavier than standard bottom brackets but much larger bearings and much longer lived than a standard bottom bracket. Link is below.

http://www.compasscycle.com/SKFbb.html

If you are a weight weenie just get a SKS for the stoker and a standard one for the captain. The doubled force on the stoker crank wears bearings faster. No matter what crank you use, failed bearings can end your ride and we don't like to walk. Our total weight of bike + riders is about 195,220 grams so I am not worried about 80 more.

Wayne