On the NPS site for the Guads, the forecast in the next few days is lows in the 20s. That is around 5,000 ft. You could be camping at a higher elevation with lower temps. A 40 degree bag won't kill you, but you will probably be very uncomfortable with little sleep (been there, done that). I would not do it, but instead take at least a 20 degree bag.
If you still were going to do it,
1. more insulation on your torso/head/feet
2. full length sleeping pad
3. hot water bottle inside sleeping bag (risky)
4. hand/feet warmers
5. bivy sac (~10 degrees more warmth)
6. liner (~5 degrees more warmth)