It can be done and the benefits power wise are there, but it is a load of work to get the 24V working in a 116 GTV...

The 24V 3.0 168kw@6300
The 12V 3.0 141kw@5600

 

I personally feel you'd be better off focusing on getting a bit more out of the 3.0 12V. Some sportier cams, cylinder head work and CSC's larger-bore manifolds and downpipes should give you a healthy increase in power without sacrificing driveability. You can also buy raised compression pistons from the likes of Wossner. If that doesn't satisfy you, there's then the options of overboring - I've not heard of many 3.2 12 Valves (would that be achieved simply by installing a 3.2 crank and rods?) but there's a couple of places in the UK who do 3.5 conversions.

And as Gertie said above, installing a 3.0 12V will require a lot less work than installing a 24V. It all boils down to what you fancy really! If you haven't got it already, I recommend Jim K's book on tuning V6 Alfas, it'll give you plenty of food for thought and it's an excellent read.

Alfa Romeo V6 Engine - High Performance Manual (Speedpro Series): Covers GTV6, 75 and 164 2.5 and 3 Litre Engines - Also Includes (not for front wheel ... Advice on Suspension, Brakes and Transmission: Amazon.co.uk: Jim Kartalamakis: 9781845840211: B

Hope that helps.

 

Having messed with 12V motors in the past, both aspirated tune and supercharged, I'd say go 24V. As late a version as you can too. Parts for the 12v (and early 24V) are getting scarce, the valvegear is always wearing out and even when perfect tends to be a bit noisy (if that bothers you). The 24v is always going to breathe better. Find a 166 CF2 donor and use that would be my suggestion. Yes, lots of work to make it fit but well worth it.

 

I don't mind the work, I have a machine shop at my disposal as well as an electronics development lab, so getting the 24V in doesn't really bother me workwise - time wise it's a different story!

Anyway food for thought I guess...