Cool find on some nice old Mopars. Even though the thread is 3 years old, it is nice to get updates. It would have been nice if the owner would have been willing to let them go. Well, now you have their contact information. It might prove fruitful to call them all along and see if their position has changed. Who knows, they might change their minds and sell them to you just to get you to stop calling...
As far as the legality of going on the land to check out the cars, that is a call the individual would have to make. While it is technically tresspassing, in the end, it is your call. IF you know the folks and the area and as you said they are friendly and you felt comfortable, then do it. I think a smile and friendly demeanor on your end will get you a long way towards making friends with the owners. If you are not sure of the folks that own the cars/land, then don't get on the property until you have their permission. I have done it both ways. When I know the folks and the area, I have ventured onto the property and met new friends, even drug a nice '68 Bee out of the woods one time. If I don't know, then I don't go. I will contact them first. In all my travels chasing these old cars, I have only been refused entry 1 time, and that was over the phone, not while I was on the property. I think you can kinda get a feel for the situation and make a decision based on all the information you have. I know, it is still technically tresspassing, and I am not advocating that, I just think for the most part, folks don't mind if you are up front with them. I have always left them with my name and contact information when I leave. That has lead me to more than one car just by making those connections.