I see some positives, but some negatives as well. I know Johnny was inconsistent when he signed, my best advice would be to contact Bill Miller at Johnnycash.com
I sent Bill and email and all he said was "No." I asked if there were any markers that led him to believe it wasn't and he said "years of experience." So I guess technically I have his opinion, but it didn't seem very helpful as I don't know why he feels it's a fake.
Bill is usually pretty solid; solid enough where even if this WAS authentic, just the fact that he said no would steer me away from it. Cash autos aren't too rare or expensive though so I wouldn't sweat it. Like I said though, it does have positives and negatives in my opinion, though im not well versed in his graph. Usually though I've seen his H stem fall below the J, and the J formation is ever so slightly off. Cash is so hard because he was all over the place, but Bill would be the opinion to take, unfortunately.