Lots of reasons your car could be slowing up. Lack of head and intake flow, small carb, fuel issues, ect. My Indy headed engine that ran a best of 5.98 exactly what this engines best was and ran was out of snuff at 1000 foot. And it still ran 9.38@138mph. It’s problem was a solid lifter cam and an 850 carb. Now I have the same .660 lift but a roller cam and a bigger 1000 cfm carb. With my 4:88 gears the old combo was out of guts.