Im currently in the process of rebuilding my alfetta but at the same time in going to be doing a v6 conversion to the car, im just stuck on deciding what size i should put in, currently im looking at the 2.5 24v 156 due to the engine being widely available.
I will however post some pictures on the build/progress
3.0 is offcourse always nicer then a 2.5.. But probably also more expensive and harder to find. 
The 190bhp from the 156 will definetely put a smile on your face. 
And if you use a short final gearing it will be a nice screamer also. 