Have you looked on the backside of the glovebox lid? My 57 GMC 370 LCF has all kinds of info on a tag riveted on the lid. It tells me mine is a 270 engine, cab 1762,Tranny GM type SM420 4speed and lots of other stuff including rear end,spring leaves, ectra. I have found out that GMC numbers are nothing like Chevy.
Wayne.

57 GMC LCF 370
55 John Deere 40-W
59 John Deere 430-W
2000 GMC 4x4
56 John Deere 420-W
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