Did you find the Parts id Plate on the glove box door. This plate will tell you all the important info on the truck.
The B7000 N model ran from 1960-1963 and had four series. The first number after the N is the series number. The series number will tell you the year Ex 1=1960,2=1961,3=1962 and 4=1963.
eBay has the 1960 GMC MODELS 5500-9000 FACTORY SERVICE MANUAL (X-6034) for $13.99. that manual will cover the first two series 60-61.
X-6034
https://www.ebay.com/itm/36429109921...Bk9SR9Lfuam5ZA
Depending on the year of your truck, you might need one or both supplement manuals X-6234 (1962) and X-6334 (1963).
X-6234
https://www.ebay.com/itm/19636602075...Bk9SR5zs76i5ZA
X-6334
https://www.ebay.com/itm/39276868023...Bk9SR5zs76i5ZA
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Parts id Plate