The vin should be located by the steering box.Waht year cab did you use on your 80?Plus do you happen to have the original vin number?If so just take the vin tag on your 80 cab and transfer it to where it was on your old cab to your new cab, take the vin from the new cab and put it on the old 80 cab and cut the cab up and scrap it.Thats what i did to this cab i got.As long as the cab is chopped in pieces its considered scrap and not need of a title when you take it to the scrapyard.
thanks
pat