In my opinion.... There is nothing wrong with using a donor body shell to repair a rusted out car. The only thing that makes it a no no these days is re-birthing.... Re-birthing is using the ID from a badly damaged vehicle on a stolen vehicle... This is totally different to re-shelling a vehicle legitimately.

up until the late 80's or so a complete body shell could be bought from car manufacturers as a replacement part. Most panel shops that repaired a damaged car using a new body shell would swap the tags from the wreck to save the owner the hassle of having to go to the rego dept. and change the details. Technically this was an illegal practice but was commonly done.

it is quite possible you could by a car that had a body swap done to it 30 years ago using a new replacement body shell.