[G&R's57GMC]

If I’m not mistaken the V.I.N. Is only on the chassis on the GMC in two places on the top of the left frame rail .
One near the center line of the front axel that would be covered by Mustang II top mount . The other is on the top of the left frame rail centered between the cab mounts .

If you were to change the chassis you would loose your V.I.N. That matches your paperwork.
Food for thought .

Now if this were a Chevy the V.I.N. Would be in a third place on a tag attached to the driver’s side A pillar .