Tung oil is great, but a lot like linseed, needs scuff sanding and reapplying occasionally and you'd need to sand your previous treatment well for it to work, it too is best on new or unfinished wood .... I don't think you'll find a permanent clear finish [like marine varnish] that will work well over linseed, need to sand that enough you'd cut into the black and so no longer have the original appearance .... [by the 60's I don't think GM was using lamp black any more, but some type of paint] .... sounds like you might best scuff what you have and rub in a complete coat of linseed oil
Bill
