They did not typically run more than 7,200rpm back in the day and would have run best in the 4,000 to 7,000 range. Remember they did not allow roller cams, roller rockers, or stud girdles and did not have the modern cam profiles we have today. Also, they had to run for 3 hours minimum (typically) plus Sebring and Daytona were 12 and 24 hours respectively.