Wheelbase is not encoded in the VIN.
You'd have to know the model code:
C30903 = pickup = 131.5"
C31003 = cab chassis = 135.5"
C31403 = cab chassis = 159.5"
You could send your VIN to the GM Heritage Center and they can provide the original invoice.
K
From the Heritage Center: In addition to visual media, this archive is also home to our company’s collection of vehicle build documentation, dealer invoices and/or build sheets, which contains information about individual vehicles produced by GM and exists on various digital and microfilm formats. The documentation is not available for all vehicles and dates only to 1977 for most brands. To request vehicle documentation please email
vehicleinvoice@gmmediaarchive.com or call 1-734-261-5086.