I understand from the above information that you are interested in becoming an IAS officer and such an ambition is always welcome. Aiming high is always better but one should live in present. You should be a Graduate in any discipline to become eligible for appearing for Civil services examination. So, first decide yourself which stream you are interested in , Science or Engineering?
The cut off mark for the JEE Mains examination for getting qualified for Advanced and to get a rank for general category is 100 while you got 98 only and you are saying that you are sure of getting a seat in any NIT. Then it is imperative to understand that you have some concession under OBC, SC or ST or PH. As such, you can take it granted for securing a seat in a NIT. If that is the case, I advise you to opt for B Tech as it is always a dream for a XII student to get an Engineering seat in one of the Premeir Institutes of India It has no comparison with your securing a seat in B Sc. in Delhi University, frankly speaking, unless you have an aversion in studying B Tech.
You can always prepare simultaneously for the Civil Services examination while pursuing your Graduation by going through various books and journals so that you can get geared up for the same after completion of the B Tech or B Sc. So aspiring for IAS is different from you academics and focus first on your academics.