The Alfa Spider is a flexible unibody prone to rust. by stripping it to a bare body you can cut out the firewall, the transmission tunnel, and the lower legs (the front suspension mounting points) and replace that with a lotus elan style backbone frame. allowing you to fit it for whatever drivetrain you care to fit. you can also fit it with IRS. The Mazda Miata is the same wheelbase, and weight distribution as your spider.
and the Monster Miata components, (custom driveshaft and axles with a ford 8.8 irs center) will take much more than an Alfa V6 can put through it.
You end up with a totally rebuilt and stronger car.
Now if your really crazy, you can even fit an entire GTV6 drivetrain, but the Miata/Alfa JTS V6 hybrid is less trouble and quicker.