If you are really fired up for big south, you could do it in a long day from the source if you didn't want to do the overnighter. Levels are high (fun) right now, but not rediculously high.
I'd plan for 7-8 hrs on the big south proper if you didn't have anyone in the crew who knew it and you scouted a lot. Not sure how long the upper part from the source would take. I know the run, and took 6 hrs with a crew of 4 when we scouted basically everything due to flow changes and wood concerns. If you were busting ass, you might get in the 5-6 hr range as well.
Saw a truck with NC plates at the take out who was supposedly doing it from the source as an overnighter.
Go run spencer heights and leave a car at big south takeout and book it over to rocky mountain national park that night to get as close the the trail ridge road put in as possible. Put on early (7am) and you should have enough time for the day trip. You could camp out at big south that night, maybe hit some poudre stuff the next day, and run back to rocky mountain the next day to pick up the other car. It will be long but worth it. I wouldn't do it unless you are in good physical condition though. The high altitude and long day can kick your ass, doubly so if you are coming from lower altitude.
There is a hiking trail along the entire run. If the shit hits the fan, bring the headlamps and start hiking to the car.