That's a tough one. There are lots of his sigs on the market that look similar to yours, but a large problem with that is that he had secretaries signing his mail for years. What is the date on the bottom of the diecast? I've seen a couple different times when someone was trying to sell an "authentic" signed diecast for top dollar where the diecast was produced after he passed.
I live in Watkins Glen and was fortunate enough to see Dale quite a few times over the years and he would always sign for me... he once almost ran me over in the airport parking lot and must have felt bad about it because he then signed everything that I had.
Hey Brian I was wondering if you had an email address that I could send you a couple of pictures of this car. It was manufactured in 1999 and I found a sticker on the bottom of the card says authentic autographs 1853 Thanks for or time Tim