Thank you guys! Yes, the wheels don't fit perfectly, maybe it's my bad but I just can't push it more in as the whole thing feels so fragile to me. Anyhow, the tires aren't perfect by the way as originally Fiat offers this type of rim in 2 different size: 185/55 with 15" rims and 195/45 with 16" rims. The tire markings say 215/50 15" on it and this size of rim/tire combo is not available from Fiat, maybe some aftermarket. Never mind...
Cobra11, as for your question: difficulties during the build? Well, the one-piece glass is easy to sit in place on the front end but doesn't seem to fit at the rear end, and during the assembly of the bodyshell and finished chassis the body is stretched so watch out for the previosly glued rear and front bumper not to break off. And that's it, otherwise not a bad kit, but there's a big difference in the details of the front and rear seats, and replacing the stock rims requires so much work because of the size (mostly the width I mean) of the stock rims.