I don't usually factor a motion ratio on a live axle rear or beam axle front. Just use 1-1. Typically they have more unsprung weight which can throw you off if you don't factor that in. Sounds like you are on the right track though.
Sprint car huh, I would love to take one of those for a few laps sometime. I had an opportunity to drive a midget for a friend of mine, but he got out of racing before we could make it happen.