The vin should be on a stainless plate riveted to the driver side door jam below the striker, if its not there then someone probably removed it. It was also on the two paper tags in the glove box but those often get removed, ive also seen the SPID on under the hood on the passenger inner fender instead of in the glove box. If your lucky it will still have the build sheet under the seat between the springs and foam, if its there you will be able to see it on the passenger side; just stick your head down there and look. You will have to remove the seat to get it out because it is very brittle, pull the springs away from it and gently pull it out.
Did you buy the truck without a title?