gear change....possible gain?

This was my set up and it worked in my 67 Dart with a healthy slant 6.
Using a 5000 stall 8 inch converter with 26'' tire and Sure Grip 3.73's ran 13 teens in 1/4 mile.Only change was to 4.30's and a spool car ran 12.90's.Approx 2 tenths or so gain.