As I recall, Peter Krause had a post (possibly on the Yahoo AR Racing site) that said the 5.50 would not fit the Giulietta Spider, I might infer that that's true for the Sprint as well. I have the 5.50-15 Vintage TDs (6 in rims) on my 66 GTV, they fit well.
I'd check with Peter to confirm
Wil Painter