Trying out 400km in a day at around 95+F this Friday, what should be I taking caution of? Should I go with clips?

I'm from Dhaka, Bangladesh. I'm participating on a group effort to complete 400km this Friday. It will be a mixed weather, partly cloudy, partly sunny. When sunny the temperature may go well beyond 95F. One thing I'm not sure of is if I should go with clipless pedals or not. I've done 100km rides with clipless ones but wondering if it will be wise to go for the 400km with clipless as sometimes I feel my left foot getting numb while clipped on for long distance. What else should I be taking care of? (I know that hydration is very important and already planned for it)