Help with the VIN and panel alignment ... I think it's a Frankenstein

[ChrisLCrawford]

I purchased a "1981 C10" a year ago.  It was good from far and far from good.  Fast forward 1 year and I have it at the body shop, having it stripped, grinded, cut, beat, etc. 

The biggest problem we are finding is that the rocker panels are approx 3/8" recessed from where the front fender stop and where the bed starts.  This also creates an uneven gap around the door. 

My 1st thought was that the rocker panels had been replaced by someone who didn't know what they were doing.  But after closer inspection, that does not appear to be the case.  I've almost convinced myself that the truck has an '81 frame, front cap, bed, and possibly doors, with an older cab. 
*note - I am also basing this theory on the multiple under colors that are revealing themselves during the above listed stripping, grinding, cutting, etc.

My question is, the VIN # should follow the cab...correct?  (If I'm wrong, please let me know)  My problem...I can't find the flipping VIN #!  There is not cutout on the dash for it to be viewed from the windshield.  There are not stickers or stamps on the doors.  And I can't find info that tells if there are differences in year models that would cause this kind of crazy issue.

I'm attaching images, but will get ANY photo needed by anyone who replies, if it will help me figure this mystery out.  So please ask.

[hatzie]

The VIN follows the cab in most states.  If you have a clean title to the cab then usually you're OK..  unless the frame belongs to a stolen vehicle.  They will probably impound the whole rig if a major part like the frame is stolen.

73-77 had VIN tag on the doorjamb. 

78-86 CK & 87-91 RV series had cutouts in the dash pad and the VIN tag rosette riveted to the metal dash sub-structure.  The complete VIN should also be on the rating plate on the doorjamb or door.

Pull the dash pad off and look to see if the VIN is there and just covered up by the pad.

There's usually a covert partial VIN under the cab on top of the RH frame rail.
If the engine and transmission are original to the truck they will usually have the six digit sequential production number from the VIN stamped into the same location as the three digit build code and production date code.