I would try and get a copy of the current SFI specs on a NHRA legal cage, it will have the correct minimum tube diameters and wall thickness for both C.M and M.S.
The main hoop bar on a roll bar is 1 3/4, on a cage it is 1 5/8 is what I remember now
If your thinking of having it certified later to faster than 7.50 ET in the 1/4 it has to have a all C.M. cage, not mild steel is what I remember a friend finding out the hard way later after having his new race car built with some M.S. bars in it