1970's Dirt Track Car

I'm looking for some help tracking down a car that has sentimental value to my dad. Unfortunately, the details he provided me were vague. He says there was a dirt track car that raced on a national circut in the early 70's. The mechanic/owner of the car was a guy from St Paul named Sonny Heck. The car # was 38 and he believed it was sold to a guy near Indianapolis.

Any help is appreciated. If anything, can someone give me the names of the national racing circuts of the early 70's?

Thanks!