I take it you meant that the 12 valves you had were less torquey than the 24 valves? What can I expect to pay for a 12 valve vs a 24 valve 3 Litre?
I take it the 3.2 is a modern engine in comparison??
The 3.2 I was referring to was the proper Alfa V6, as fitted to the GTV, GTV Spider, 156 GTA, 166, 147 GTA, and GT 3.2
The 3 litre 24v is very torquey low down for a multi valve engine, although the 3.2 uses different cams and tends to produce slightly less lower end torque.
The easiest place top get a 24v 3 litre is probably from a 164, the version fitted to the 3 litre super/super lusso is 210bhp and the 24v cloverleaf version is 230bhp (But has a bit less torque).
There are some small detail differences between the 164 24v and the GTV 24V etc, but I take it you will be making custom mounts so it should not be too much of an issue.
Another good engine option IMO would be a 5 cylinder Fiat coupe turbo engine. They are cheaper and easier to get than Alfa V6's, but it is physically longer so it may be harder to fit it to the car.