Fuji across is a bad film for underexposure. It's critical at that point.
I use/rate it at 50 asa in combination with rodinal or pyrocat hd,
So 400 would be an overexposure of 3 stops.
That would mean that your zone III (middle gray) is placed in VI.
Be aware that your original scene zone II,IV and V will not contain detal when developed with rodinal or pyrocat-hd.
overdeveloping your film will not get detail in these shadow regions.
The only thing you could do is choose a developer that is capable of using the fuji across at a iso value > 100. The higher the better for your case.