When I flew in the USA, our company had an ex-Air Canada DC-8 equipped with the extended rails (to the F/O's seat). I think it was a 50-Series and it was used as a Combi. I know George wanted to fly it as a 2-pilot machine but the FAA wouldn't let him.
A friend from the Great White North who flew the DC-8 at Air Canada years ago told me that the FO would slide back and drop the crossfeed knobs on the fuel panel for takeoff. He or she would slide back again above 10,000 feet to make sure everything was feeding OK and go back tank to engine. And, I presume later set up for landing with the crossfeeds open in case of a miss.
I was guessing that the two pilot Air Canada DC-8's were only the 70-series freighters but since you saw a short Combi, maybe they went to two pilots, no FE, while still in pax operation.
I can't seem to find much online about the AC two pilot DC-8 operation but I have seen the extended FO seat rails in a -73F years ago. At least I think I have...
While I didn't operate DC8s I rode jump seat on many and they weren't much different to the B707 including having similar engines and systems.
As far as Boeing and Douglas systems being about the same to the FE, I'd beg to disagree. The early Boeing jets were much more electrical in my view. The DC-8, and to some degree its descendants, had most systems operated by long mechanical cables and electrical failures were not as big a deal. The DC-8 was affectionately known as the 'Cable Car'.
The DC-8 had the cash register, the sugar scoop and the suitcase handles as nicknames for cockpit controls. I believe the split suitcase pitch trim handles are still on the MD-11.
Many of the professional flight engineers who taught me the plane decades ago still had bitter memories of ALPA crossing their FEIA picket lines in the 1960's. FEIA wanted to make the FE second-in-command. ALPA successfully renamed the copilot and flight engineer to first and second officer to eliminate any ambiguity about succession.