Nice start, I'm going to follow this build closely.
Of course you could have a 327 in a 58 Impala in 1962 as it was the first year for that engine...but could a High School boy afford a brand new engine and "hop it up" with a new intake manifold and 6 Strombergs, that's the question...he could of course have wealthy parents...so it's possible.
The thing with the Falfa 55 is that, as many of you know, the car was modified from the Two Lane Blacktop outside scenes car, the engine in that car was a 427 L88 after what I have found out during my research about the TLB cars, the other two built for the TLB movie had 454's...the engines have the same outside apperance tho'.
But the American Grafitti movie are supposed to take place in 1962 and the Chevy 396/427/454 "Rat Motor" hadn't come out back then as the first year for the Mk IV Big Block was 1965, so the 427/454 in the TLB 55 is wrong for the time period the AG movie supposed to be from, the TLB movie takes place around 10 years later and the 396/427/454 was for sure in production by then.
So if you want to be true to the Falfa car itself it should have a 396-454 "Rat Motor" and if you want to be correct for the time period the American Grafitti movie took place you would go for a 348/409, 327 or anything else available at the time.