I'm pretty sure that the exception that allowed you to upgrade AF trans-Atlantic flights (booked via DL codeshares) with Delta miles/PMUs has been closed.
Further, as a Delta SkyMiles member, you'll never see the Air France First Class cabin. For some stupid reason, Air France has decided that Delta SkyMiles are not "worthy" of redemption for Air France First Class and sadly, Delta hasn't really tried to fix this (you listening,
RealDLInsider?).
Business Class on Air France is very nice. Perhaps just a cut below Delta BusinessElite, depending on what you value in Business Class. The food is very good, but it
IS French, which puts some people off. I'm told the wines are excellent. I personally despise the Mimbari-bed seats in AF business class, and the 2-3-2 configuration of AF business class on many planes.