I looked through some old USAC yearbooks to find some answers.

On May 28, 1979, Tom Bigelow won his 43rd USAC sprint car race at Dayton Speedway to tie Larry Dickson for the all-time lead. He broke the tie on June 16, 1979 at Salem Speedway. I don't know for certain how many starts it took, as USAC only printed the top 12 finishers through 1980, but Bigelow ran 31 of 39 races that year and finished the season with 339 starts.

As noted, his 52nd and final victory came on July 30, 1981 in Beaver Dam, Wisconsin. He made 9 further starts that season, ending with 376, so his 52nd win came in his 367th start.

The most recent yearbook in my possession is 2005. At the end of 2005, Darland had 20 wins in 304 starts.