Roland,
I have no idea where in your car the 2 x 3 subframes are going but the main hoop and door hoops of your roll cage should be tied into the rocker panels, not any subframe that's running nearer the centerline of the car.
NHRA still gives a weight break for 4130 and you can save weight with 1.625" x .083" tubing. If you plan on running your car with most any road race/track sanctioning body here in the US, .083" 4130 tubing is only allowed if the tubing is 1.625" OD. And, BTW, DOM tubing of the same dimensions is allowed so the extra cost of 4130 doesn't get you any weight savings.
EDIT: The above road race tubing dimensions are for a car weighing 1,700 to 2,699 lbs. without driver. If your car weighs in at 2,700 lbs or more then you'll need 1.75" x .095" or 1.625" x .120" wall tubing.