DL781 looks just fine, about 1/3 empty plus quite a few no-shows expected.

All of the YYZ-ATL flights also look good, but I'd arrive early to be safe, especially when taking a ZED. I was helping one guy on this site, and there was a cancellation out of DAL that made it unexpectedly hard to get to IAH. I had something similar getting out of CLT (thankfully I had plenty of time and both UA and US benefits to work with to get to ORD). If you go early, you're less likely to have those cancellations. If you really need that extra hour of sleep, however, 5045 is the most open flight right now with over 30 seats left (many being in F).

Check back for more YYZ-ATL loads closer to the day of departure to be sure. I'll probably lose my DL site lookup abilities when I leave my current job on 5/3, but mrs767er is an excellent resource for DL loads.