Try looking on top of the frame on the drivers side of the engine bay. It should be stamped right there in easy view. Also look for a stamping on the front frame horns, on the outside of the rail. You may find the year of manufacture. It might say something like "60" with a circle around it. Don't freak if it's an earlier year than what your truck is. They just used up the older frames first. (Not clear if your are looking at a K20 or not but, you may look at the spring hangers-the stamped pieces- look very closely and you may see a GMC script on them if they are clean and rustfree. 4x4 only and yes, this applies to Chevrolet 4x4's)