You can do it. It's easier if you have a 155 V6, so you can use the V6 ancillaries from the 155 to move all the thermostat housing etc, to the battery side of the engine. A GTV V6 engine or a 156 V6 engine should fit better as they all (I think) have the thermostat housing on the battery side, but the 164 and 166 is not the same. The only problem with the 2.5 24V V6 is the drive by wire thing. I wonder if that blue (UK) 145 2.5 24V has the fly by wire thingy.
I think a compromise would be an engine from a late (like '96,'97) Spider 3.0 12V. That way you can use the 155 V6 standard exhaust manifold, and I think, the thermostat etc all are in the right place.
If you have a good fabricator or you can weld yourself, I don't see a problem with any engine going in the 155. May take some work, but anything is possible.
But I think the easiest is still a Spider 3.0 V6 12V into a 155 V6. If you have a 16V, may as well turbo charge it and don't bother with the V6. Up the boost and say bye-bye to the V6ers.