Yes if you were in the top 20 percentile of your 12th board examination in the year which you have given the 12th examination i.e. in 2017 according to you then even after you haven't got more than 75% or you are not in top 20 percentile of your board examination, you will be eligible for the NIT counselling and can get seat in the NIT college through your JEE Main exam marks. 

In order to check that there hasn't been any change in the policy this year, you can email your same query on the official email address of JEE Main administration and get rid of this tension with the confirmation mail from their side.

Feel free to ask if you have any more queries. All the best!!