1955 and 1956 Chevrolet have the same front seat. In 1957 they changed the seat a bit.
I remember a friends kid had a 56 and it came with a different year (either 55 or 57) bench in it and that (wrong year) seat made him sit quite a bit higher than he really should have. He actually liked it because he figured he looked like a HUGE dude while driving his car with his head almost hitting the headliner .
From the above post, it sounds like his car had a 57 seat in it so there appears to definitely be a difference between the years.
Best of luck in your search
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Actually, in real racing ... it IS how fast you went.
55 and 56 had different seat tracks. When if your head bumped the headliner of a 56 4 dr ht they would replace the tracks with 55 models at the Chevrolet Dealer.