To the best of my knowledge all 155 v6's came from the factory as 12v's. Certainly the 24v 2.5 from the 156 will fit without problems, unsure about the 3.0. Saw a 145 at last years Spring Alfa Day with a 2.5 24v in it and very nice it looked too.
Peak Alfa have converted a 155 to a 156 24v, was a narrow body 155. it has since been converted to a 164 3.0l 24 valve. This is a race car but looking under the bonet I couldn't see anything missing. Roger is a very decent bloke and might be worth ringing as he could give you an idea how difficult it would be. His web site has his details.