A front air dam, will help add weight to the front BUT, you have to have balance!
With a fast back car, where the roof line drops, the camber of that line, causes low pressure at that point and will make the rear wheels light. You must maintain a rake, as I described above. If the rear gets light, you'll need a spoiler, to offset the lift of the roof line. 3" tall would probably be enough. Both a flat air dam and spoiler will add drag.
Front and rear shocks and spring rates are important, too. If you get out of shape at 100 mph or more, you won't be able to get control back, unless the spring rates are balance. In a drag car, that can be a problem. The front spring rate needs to stiffer than the rear or, the back end, will beat you, to where your going and the lift of the fast back, will lift you right up and roll just like in NASCAR. Set up this way, the car will feel very stable at speed and go faster.
Lowering the car will help, too.
Actual road racing courses are carefully made and maintained to avoid dips and rises that can set a car sailing with all 4 tires off the ground.
The straightaway sections of a road course, will be fairly good but, can have dips in them. The faster areas with curves can have places that have a hump in the curve, Willow Springs, CA turn 6, for example and turn 9 at Riverside (no longer there). If your wheels aren't straight, while going over the hump, you'll find yourself in the dirt. Formula cars, won't generally roll in those spots but, higher CG stock type cars, will.
First rule, keep all 4 on the ground!
