1 3/4" Headers Verses 1 7/8" on a Healthy 408....Is There a Big Difference??

years ago, I changed from 1 3/4 to 1 7/8, length was 35 ". my car (340) ran two tenths and two mph faster. a friend of mine tested 1 3/4 vs 1 7/8 on his 340, on a dyno, he found 30 hp. vs 1 3/4. just food for thought.