assuming the tranny is stock 4.75 fd, i think the 24.5x8x13 would be the best size slicks. with the car trapping 120mph, the 24.5" slicks will allow him to finish @ 6315.789rpm in 4th. should have decent power @ that rpm.
23.5x8x13" slicks would make the car pass the finish line a bit higher in the power band - 6611.11rpm - but i would go for the bigger size for better traction
peace