Robin, the question is basically irrelevant and unanswerable. All that matters is whether you fit in the car.
The reason I say it's irrelevant is that for starters, it's an Alfa, and as such, was designed for contortionists. My spider for example, is best driven by a peg-legged pirate -- as long as the peg is on the right leg -- who sits with his legs splayed open in a very undignified fashion in order to accomodate having the steering wheel between his knees.
Now my brother has no issue with head room in the spider even when the top is up. He does have head room issues with 85% of the cars he sits in, and he's much shorter than you are. At 5'10" he shouldn't have headroom issues, but all his length is in his body, not in his legs, and most cars don't seem to accomodate that body type very well. He's not out of proportion -- he just doesn't have giraffe legs.
Go back, sit in the car and take the time to adjust the seat. After you get that done, take it out for a long ride, and I mean a long one. What feels comfortable for ten minutes isn't after an hour or more.
Also remember that different variants/years of the same car fit differently. Ask Selespeed about his adventures in trying to find a spider that fit him.