Name two USAC champions who later on IRL Indy Car titles?

Tony Stewart and Billy Boat

AJ Foyt Jr. three USAC stock car championships

Tony Stewart.

From Jim Morrison
I was thinking of drivers, but ... now name one more other than Stewart, Foyt, Billy Boat.

Tony Stewart & Dan Wheldon

fittapaldi and kanaan.....??

It's Wheldon... some sort of USAC/SCCA championship.

In addition to an Indy 500 winner, two NASCAR stars, and the nicest driver you'll ever
meet (who truly defines the word champion) other USAC champions in 1999 included:

Bud Kaeding - Western Sprints
Marc DeBeaumont - Western Midgets
Michael Lang - Speedrome
Ron Smoker - 16th Street Midget
Josh Wise - TQ Midgets
Tyler McQuarrie - Russell Pro Triple Crown
Nathan Smith - Formula Russell

I would have to say that 1999 was a very good year in USAC.

Yes - Dan was an F2000 Champ when it was under the USAC banner