You said it best, it all comes down to trust. 68tpls I do respect your opinion, but here is my idea.
I do plan for him to have the keys when he is 16. He won't be watching, he will be doing the work on this one. I think him having hundreds of hours of his own time and all the money he is able to earn into the car helps to make him more aware of what he has to loose. If he screws up, he looses the car. We went through people feeling the same with my oldest sons Bronco. It is odd how giving your kid a cheap newer car is viewed totally different than a classic with the same value.
As far as unsafe. They it is a solid car, will have disc brakes and a updated suspension. Sure, no air bags but you cant keep them safe from everything.
The skills Ben will gain from this project will be irreplaceable. He found the car, and I will assist him to getting it on the road. But it is his.....Unless he looses that priveledge by screwing up. (tickets, curphew, grades, general rules)