Pappajams link is the best bet for info. That site is the dog's bol locks for old Alfas and has many stories stories and attempts listed (search first - they're a cranky bunch).
I considered the V6 for my duetto, it's a bugger though (transmission, fit, mounts, exhaust).. the 2.0L twinspark or the turbo!! is much simpler (and the route i wlil take with my next 105).
duetto turbo
here you go
the thread Marco/Hambur half way down page two