USAC allowed rear-engined cars in sprints in 1973. Sneva won, I think, six features. Others also won. They were outlawed for the '74 season.

Duke Cook ran a roadster at New Bremen in July of 1974, but stuck it into the wall qualifying. Jeff Bloom won at Salem in the Lyle Roberts roadster, and I believe Marvin Carman ran a roadster a few times, and won. I believe these were both in 1975.

By 1976 or '77, the Genesee Beer Wagon wheeled by Sheldon Kinser came along followed by the Jet Rod Engineering car driven by Steve Chassey. Among others.

USAC next allowed upright sprinters to run with more of an offset and it all evolved from there.

Pavement racing in USAC was interrupted in the mid-1980s when Dayton Speedway closed and Salem Speedway was hit by a tornado and closed until 1987. A whole new generation of cars developed after that.
Jim Morrison