Bob, sorry to hear your Grandpa is not here to help you with this.
Disclaimer:This is not intended as legal advice, just things to consider before asking your legal reps for advice.
If you are the Successor Trustee or Executor of Granpa's estate, you could have a fiduciary responsibility to obtain a reasonable market value for his assets. It sounds like that is what your cousin and you are attempting to do.
It is easier with newer cars. You use Kelly Blue Book. Optionally take it to a dealer for a written offer and then decide how to sell it.
If you are the Executor or Successor Trustee, depending on the "Beneficiaries", it may be best to obtain a written appraisal of the car. It only takes one Beneficiary (or their spouse) to turn things into a legal mess. There will still be "fix up", selling costs and possibly holding costs to obtain the price of the appraisal and even then, it might not be obtained. Depending on the State, there may also be safety and smog compliance costs as well as registration catch up requirement costs.
There is a price "Guide"" for new and older cars called NADA.
http://www.nadaguides.com/
For older vehicles, rarity (350 2 bbl is not rare) and condition matter.
Without images, especially of the rust prone areas, no one here can give you a real guess. A major difference of the orange 69 above is that car is a So Cal car and likely has never seen snow. It's frame appears very solid and there has been sheet metal replacement. With a DE car, there is a chance for more rust. The '70 that I purchased 18 months ago has 97,000 miles on the odometer. However, is that 297,000 or 497,000 miles?
Bob, the easiest way to post images of the car is to use the GARAGE feature on this site.
Look to the right of this post and you will see a link to my GARAGE and info on my '70.
To set up your GARAGE,
* Save the images of Grandpa's '70 in on your computer.
* On the top menu bar click on "USER CP" user control panel
* Then on the left menu, click on MANAGE GARAGE
* Then click on ADD VEHICLE and follow the instructions to add images and text. You can add three images in each section, yet for now add images for review.