I tend to stay away from the valuation tools unless no "actual" sales results are available. I like to go to E Bay and enter a car and select "sold" search criteria to get realistic sales results. Obviously there are other better ways of getting values- its just quick and easy. In the case of 66's there's been a number listed but no actual recent sales history (probably due to the lofty asking prices).
I've seen running driving NOM 66's in 2017 with asking prices in the 90's (in Hemmings etc), contemplated buying a mostly complete but apart one needing full restoration in the 60/70k range and know of one without a motor or trans with cosmetic exterior fire damage that sold in the 20's out of New England- all in 2017. I believe Brett at Cape Cod Mustang currently has a decent driver with a "asking price" of around 140k. I'm sure many will disagree but my opinion is that someone would actually get 80-90k for one that doesn't have the original drivetrain and doesn't run or drive (less if its rusty) and take home 90-100k for a very tired one that requires full resto but isn't rusted and runs and drives well. The caveat is that we've also seen bad or good ones unexpectedly bring crazy high and very low numbers- like rolling dice. Just one mans opinion based on some actual 2017 sales