Had very similar problems a while ago with my GT. It's because the seat belts are long at the bottom. The only one that I could find to fit was an isofix (I know your car won't have isofix fittings) Britax version. The base is small so the seat belts wrapped just the right amount up (I did try with seat belts, and with a classic mimi, belts were very similar, even though in the GT I used the isofix fittings) meaning no buckle crunch.
This is your kid(s) you're talking about here, so don't take any chances.
Found an iron on patch on ebay to alfa-fy it too.
Sold the seat to another Alfa owner last year. Do have another one, but it's a bit girly, same model but more of a dusky pink & grey in colour.
Best advice is to find a store, Mothercare, Helfrauds etc that will let you try lots of different ones. if the problem is what I imagine, then look for the seat that sits the lowest, this should give you the best fit.