Should be plenty. If immigration is backed up, there is a short connection line.
Have you entered tye US previously using the same ETSA? If so it will be really quick.. Even if 1st time shouldn't be an issue.
DL will rebook you if you mis the flight. While SQ is notva partner, DL will rebook if youre coming in on a separate ticket since the issue was the change in flight. (They also will do it regardless of reason, if both tickets are ST/DL partner)