Secured Offer but I have a felony assault charge that was dismissed/expunged. How do I disclose this information?

3 years ago, I was charged a felony assault after a bouncer at a bar followed me 100 yards from the building and was threatening me, etc... The charge was felony assault because he lost consciousness, but the cops understood that situation was self-defense, so they took it easy on me and let me go after processing me. I obviously wasn't going looking for a fight as I was leaving the bar and he followed me. It was dismissed from the courts, (I never even went to court for it) and have just been granted the expungement order on the charge a couple of weeks ago. But that is a long process, and it will still show up either way.

I have received an offer from a middle market M&A firm in NYC and I just want to be prepared on how to deal with the arrest coming up in a background check. My gut reaction is that because they are a smaller firm, they won't even consider putting the liability on themselves of hiring an analyst who was arrested with a felony. Hoping that I am wrong, as an offer substantially changes my life and because I worked so hard to get to this position.

When I get the contract, should I reach out to the person I last interviewed with (he is the MD who decides if you are a fit for the firm and called me with the offer) and either ask to meet in person or zoom to explain. He was really interested in who I was as a person, so I feel as if my best shot is to explain the truth about the situation and he will possibly understand. I was defending a family member and was trying to deescalate the situation, but I had to defend myself. 

I would appreciate any help or insight. Thanks.