UPS has 13 ex-Delta DC-8-71's. UPS does not have any ex-United DC-8's, happily. United had a feature that no other operator went for, which made their airplanes a pain for second tier and beyond operators. The lower baggage compartment doors on all (I believe) United DC-8's were on the centerline of the belly. They required a special container which was shaped in the cross-section of the compartment with a curved belly. The container was raised into the compartment with cables and slid fore and aft. The container transport carts had a fabric (canvas??) bottom on which the curved bottom of the container would rest.
Sounds good in theory, but once wear and tear occurred, the cables would hang up, the pulleys would break or jamb, a container would jam kitty corner somewhere in the compartment. But they used them to the end. Second tier operators would bulk load the bellys and going in through the belly door (unaffectionately known as the bomb bay door) was a real pain in the posterior (make whatever substitution for that term you would like). And a cargo shift would, at times, end with cargo sitting on the door.
Those planes fly for Air Transport International/BAX Global. I had the pleasure of working on those particular aircraft for 6 1/2+ years. I love the DC-8, but you're right, those doors were often a "pain in the posterior" if the loaders didn't secure the nets properly or managed to somehow get them "off track".
Patrick Bateman is my hero.