Don't know if you know this(or care) but they are the exact same film except the 320 has an added film base for retouching. The 400 is actually sharper(1 less film base to print thru) Unless you are still retouching on the neg, I can't think of a reason to search out 320 over 400
And they are the same asa(since they are the same film) regardless of what it says on the box