Yes, connecting in IND is easy. Its my home airport and I've taken DL501 twice in the past month or so. CBP area is small, there are two Global Entry kiosks and once you clear customs, you just go through TSA again as you would be entering the terminal to board a domestic flight. Breeze of an airport and as others have pointed out, a newer facility, too.
Your trip on DL501, assuming it is before next May, would also net Delta $75 (I think) as they're incentivized to fill seats on this new flight (started May 24) so it could also explain why it comes up as a top option.