vehicles back then were titled as the calender year of first registration, not the model year, which started the previous fall - no doubt the truck was first sold in 57 .... funny, I had a 57 panel that was registered as a 58 - it was a special order not delivered until Jan 58
do you know about the
'resto-pak' available from GM? might be able to find where/when your truck was first sold as I did
Bill