If you saw no gain or loss in the last 1/8 mile on the converter change I would look at your fuel system to make sure I had enough volume to be able to jet it up enough to slow the MPH down in the 1/4 mile first twocents scope
Me thinks something is keeping the car back from going faster work shruggy

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)