Having been the happy owner of a 1999 Spider for the past month I just found this site when searching for advice on removing the 2 rear engine undertray bolts - the so-called captive nuts are rotating with the bolts. I expect I will have to hold the nuts stationary with a ring spanner whilst I drill them out. I live near Gloucester and had a word with the local Alfa Specialist who said it was a common problem and showed me several cars there which had had the undertray removed but the rear bolts still in place around the remnants of some the broken tray. Most owners seem happy to leave the undertray off, but it is slightly noisier without it and I want to keep as much road dirt and moisture away from the engine and alternator as possible. Once I get the original bolts out I will replace them with dzus fasteners.
Anyway, otherwise the car seems perfect and makes me smile every time I drive it.