I just looked more closely at the part names section of the image and it provided a clue as to the reason yours looks different than what is shown. Your frame doesn't exactly match the dwg because the frame shown above the parts list is kind of an amalgam/composite. It combines the features of at least 2 (maybe 3) C/K body styles.
Take the body mounting points for example:
A standard cab (03 style) uses only 2 sets of body mounts.
One is the forward set (under the cab floor and outside the rails) which is indicated by part no. 23 - Bracket, Body Mtg #1. This set of mounts is common to all C/K series trucks.
The other set (Body Mtg #2) is located on the cross member shown as part no. 9 (Member, Rear Spring Frt Hanger Cross). And as far as body mounts for the 03 style, that's it.
The Crew Cab series (43) uses identical body mounts in the same two locations - and with the same design - as the 03's.
That second set of mounting points isn't even represented in the dwg.
But the 43 style also has another set of mounts - located under the rear wall of the cab like an 03. Obviously, due to the extended cab length, those mounts are further back on the frame. And instead of a x-member, they are supported by brackets similar to the forward two sets. That is what is shown in the dwg as part no. 11 (Bracket, Body Mtg #3).
So the frame VIN shouldn't be an issue after all. However if you need to know it for some reason, look all along the top flange of the RH frame rail. It could be anywhere on the rail - from the rear wall down into the engine compartment. Also there may be more than one stamping.
BTW - it won't exactly match the VIN. It's more of a frame serial number and should be similar to this - C6S215739. C is for Chevy. Since you have a GMC, your's should begin with a T. Don't ask me - I have no idea.
This is reported to be the key to decode the frame serial/VIN number:
The first ----- letter will be C for Chevrolet or T for GMC.
The second---- C for 2 wheel drive or K for 4 wheel drive.
The third is the engine description .
The fourth is Series for Chev 1=10, 2=20, 3=30 For GMC 1=15, 2=25, 3=35
The fifth is body type 4 = PICK UP
The sixth is Year. 3=73, 4=74, 5=75, 6=76, 7=77, 8=78, 9=79, A=80.
