That's from many years of letter writing, phone calls & dead ends chasing ZL1s & Yenkos years ago when it was easy. I found title records for cars that no longer existed, missed cars that still existed but had not been registered for many years. Very common for guys to buy cars and never title them. The ZL1 we ended up with was on an open title; he remembered who he sold it to. A lot can happen in 49 years.
Your only real shot is finding someone that can do an NCIC search. That might give you the last state it was registered in. If they have no record, it was scrapped or hasn't been registered in many years.