Is this the bog-standard model?
Chris:
You should be Ok with the 8+ hours you're dealing with in Atlanta. Stone Mountain is on the way to the University of Georgia - about an hour East of downtown. My only caution is that you won't be driving back on highway and when you do get closer to downtown, traffic from 5-6:30 p.m. can be very bad, both north and southbound through the city and I suspect that's when you'd be heading in. Have never been to the speedway, so can't tell you much on that.
Just rember, it's the security lines that kill you at that time of day. I'd be in the security line no later than 7 p.m., so work backwards from there.