The body swap would to me preferable to the fact if you cut out all the rusted bits of a wreck, welded in replacement panels made from todays metal, probably from overseas, that are often not a perfect replica and call the car original - either way there has been a lot of changes. At least the body was made in a Holden factory by Holden employee's.
The same goes for mechanicals, if the original is U/S or already gone then a replacement from the same era has to do. It is, as others are saying, only an issue when you go to sell it. There is always then going to be price variation from concourse completely original to replica - the market will decide where your project sits.