Note that the 1.8 engine in the HGT is a younger brother of the 2 litre Alfa lump as used in the later 155s. Might give you a few more options, people to talk to, etc.
You have to live in hope, but I doubt very much that the HGT box is anything like as tough as the Coupe 16v Turbo one, which is, in essence, the Delta Integrale box, which has extensive re-inforcing plates. Still, the rotrex will put much less strain on the box (and the entire drivetrain) than a positive displacement blower.