I started looking into this about a year ago and posted about it at the time. (I got some silly replies about using a 20V turbo engine from a FIAT coupe?!?)
But lack of spare time and space have prevented a lancia dedra turbo transplant happening (so far). I reckoned the best way to do it was to get a Dedra Turbo 8V and take the brains, manifolds, rad, hubs, discs, calipers and d/shafts and swap them over onto the 155. I got as far as phoning a Lancia specialist and talking to him about it. He felt it was an easy swap mechanically. He also said I wouldn't need to swap the subframe. At the time I had located a dedra turbo se (165bhp) as a donor, and I suggested using the mechanics and running gear from the front of the dedra and mating it to a 16V turbo unit from a Thema, but similarly to a post above he felt that as 200bhp was available pretty easily (and cheaply) from the 8V it wasn't worth sourcing a 16V..though he had a couple lying around with bent valves that I could purchase off him if I wanted.
He intimated that the 16V thema engine will mate to the gearbox of the dedra turbo perfectly as the bottom ends are the same.
It has to be the easiest way of adding true performance to the alfa's good looks without spoiling the handling.
I'm still up for doing it, but the alfa is my wifes daily hack at the moment, and she needs it to get to work. But it might still happen....
I'll keep you posted if it does.