Options for Atlanta to Port Canaveral, according to Google Maps:
1) I-75 to I-16 to I-95: 533 miles and 8 hours 55 minutes
2) I-75 to Florida's Turnpike to FL 528: 491 miles and 8 hours 21 minutes
3) I-75 to I-16 to various state and federal highways through south Georgia to I-95: 482 miles and 8 hours 11 minutes
Option 3 is technically the best in terms of time and mileage. However, it requires a lot of semi-local driving through rural south Georgia once you leave I-16. Option 2 is slightly longer, but you're on interstate almost the entire way, though you will have to pay tolls on the Turnpike.