I go into a buddies shop and see a nice Duster. Or so I thought

Another side of the 'fraud' issue.

If he has it titled & insured as a '70, it is also insurance fraud.

Title & policy should match the body.

If he is in an accident & they notice the altered numbers, they do not have to pay & could possibly even prosecute him.

If the 'donor' body turns out to be stolen, then bigger troubles for him.