My source tells me the 767 line is still up and running...willing and able to add new 767F orders.
IIRC, current 767 production is about 2/month - one each for FedEx and USAF KC-46, baring new orders that's expected to hold for several years. At least in theory, they have the capability to build 767s at 7/month so there is unused capability (although that may have been reduced somewhat when they transitioned the 767 line to ITAR about 5 years ago to support the KC-46 production).
A big problem with converting passenger 767s to freighters is that the passenger airframes are getting well and truly used up before being retired - many 767s being retired have over 100k hours.