The 427 Tunnel Port in this picture was running a dual quad setup. NASCAR allowed this in 1966 in an effort to offset the performance deficiency caused by the 426 Hemi's heads. In 1968, the Hemi was also allowed to run dual quads. In 1969, all engines were limited to a single 4 barrel.
With his brother Donnie helping by relief driving, they finished 3rd. Richard Petty won the race.
Tony Hulman formed USAC long after he bought IMS, so the idea wouldn't have been impossible. Plus, a lot of drivers including A.J Foyt, Richard Petty and Bobby Allison would go back-and-forth between the two series, so they probably would've had a good selection of big names show up for the hypothetical race.
It was basically the secondary plotline of the first Cars movie. The top 3 in points ran away from everybody else, won 21 of the 32 races run, and the final points tally was razor thin between them while 4th was just shy of 400 points back from 3rd.
Allison had the best season of his career in 1972, and it still sits amongst the greatest seasons of all-time. In 31 starts, he scored 10 wins, 25 top 5s, 27 top 10s, 12 poles, he led 4,343 of the 10,063 laps he completed, his average start was 4.0 and his average finish was 5.3.