"Per our conversation on March 25, 2002 ..." or "Per our conversation March 25, 2002 ..."
A. I would say "on March 25, 2002," rather than just "March 25, 2002."
However, I would not say "per our conversation." It is one of those stilted expression that finds its way into too much business writing.
I would say something like, "As we said on March 25, 2002" or "As we agreed..."