From personal experience two stops overexposure should be OK when processed at the rated ISO of the film (400 in your case.) They will just have to adjust the printing. Don't know about 2&1/2-3 though. It seems like it would pull OK, but you would have to find a lab that would do it for you, and in my experience the average lab can't alter the setting to something different than the ISO on the film. Someone might know a pro-lab that could pull it for you though.

good luck,