Start by joining the NHRA Junior Dragster program...
http://jrdragster.nhra.com/joinjdrl.pdf

they'll send you a rulebook which has all the cage tubing diameters, wall thicknesses and construction dimensions (Frame, Section 4)

While it may be true that a chromoly car will sell easier than a mild steel car, either one still must weigh 225 lbs minimum less driver. I've seen a chromoly car weigh in at as little as 165 lbs, so you have add a lot of weight to make class. Maximum amount of removeable ballast is 25 lbs. Maximum amount of total ballast (removeable plus permanent ballast) is 100 lbs.

If my son or daughter were going to be in the car, I'd rather build it close to the limit with structural material for his/her safety rather than to build a flyweight car and have a lot of ballast in it that will do nothing for safety. That's just the opinion of this old tech inspector, though, what do I know??

For you green people out there, there's also a spec in the rulebook for an electric powered car!!!