I dropped Engg in 2nd yr and am pursuing maths honrs. I have X-87%, XII-68%, grad Ist yr 76%. What are my chances to get in top IIMs.

Based on your academics, the chances are average to get into top IIMs for the ones academics matter. Unlike IIM B which is very strict in past academics weight age, some of the colleges/exams apart from IIMs you can apply and give less importance to academics are:

FMS
NITIE
NMIMS
IIFT

Also incase you want to check if you have chances in top IIM's based on a academics and other parameters the you can even check it through the below tool:

CAT Pathfinder

It shall give you the basic idea of your selection chances.

Good luck.