Years ago we reaced a duster with a 440 braaket motor with a M.W. top end on it with both carbs, it was about four tenths under the index to start with. We found that by shifting it higher(7000 RPM instead of 6400 RPM into both second and third instead of lower we could slow it down a tenth. If we shifted it at 5000, 5500 or 6000 RPM it would slow down maybe .030 to .050, not a ful tenth. this was before delay boxes and electronic throttle stops, a long,(1984) long time ago Adding weight is probally the better way, put it in the middle of the car between the axles