Hiii

You have a decent score, but i would suggest that you appear in april session and try to get more marks than this and you need not to worry because the best of the two will be considered. Talking about getting into NITs, the competition is very high. The cutoffs are yet to be released for NITs, and you can only get to know about your chances once the cutoffs be declared.
Being in ST category i believe you have a decent chance of getting into top NITs with mechanical engineering.

Thanks!