One stop of overexposure should be fine with neg film. I used to expose my ISO400 at 250 (+2/3) for a little more shadow detail and "insurance" that I wasn't underexposed if the meter was fooled, etc. Underexposure can cause more problems - I'd say that generallytwo stops over and one under are passable and right on to one stop over is good.

The faster the film speed, the more contrasty it seems to get so I'd probably recommend shooting 800 at 640 (+1/3), but again 400 should be OK.