A clear or not, there are many issues to consider.
The first is to do a detailed inspection of the paint finish on the hood, top and trunk.
Use a magnifer and look closely for small cracking, crazing of the paint.
Look for "rust streaks" in the paint finish, which means that it is rusting from the sheet metal up.
Look for etches, rust "blooms" in the paint finish.
Look for discoloring of the paint finish, mis-matching areas, etc.
Should you find any of these conditions, show the owner "exactly" what is there and explain the condition to them.
If none of the above are evident, it will not hurt to clay the vehicle, if any are present, I would not recommend doing so unless the owner, following your showing and explaining any of the conditions, signs off, in writting that he wishes work done.
Those older GM paints still used a "lacquer type" paint system and can be a bit touchy.
Grumpy