[jonzcustomshop]

things to check for..was it 2 cabs welded together?
the vin is in the right spot for a 64 gmc
but that is definately a chevy cowl/dash.
from the pic it looks as if the gmc vin was left spotted to its original metal... but that i was cut out about 1/4 inch around the tag, and then spotted/glued to your cab.
the correct spot for a 66 chev vin is right abovr the driver door, there should be 2 holes where the rivets used to be..

also check the driver frame rail by the control arm for the original vin.

that all said if it is titled in your name , and there is no state inspection, I would leave well enough alone with it...try to keep whatever records the previous owner had.
he very well could have had a junk gmc, and a good chev cab, and had gone through all the legal chanels to do the switch.

I have a thread titles "where is my 60-66 chev/gmc vin' check it out for locations and more info...