I'm late arriving on this thread but will answer oceancrossing's request for input from those days at Air Canada the airline that first purchased its DC-8 fleet in the spring of 1960. Called Trans Canada then it became AC in the early 60's and eventually owned a total of 42 almost exactly split between the short and long body versions. I began there in 1966 as a DC-8 S/O, retired in 2001 having done time in all three seats in the DC-8 for a total of 4000 hours on type(s).
Airbubba is, as usual, correct in his description of the cockpit but to my knowledge the aircraft was never flown with a two person crew - regulatory approval was never granted.
The company had Douglas move quite a few gauges from the F/E panel and relocated them to the panels in front of the captain and first officer (hydraulic system was pretty simple and went on the left side, gauges for all fuel tanks, 10 in the final versions, were lined up at the bottom of the first officer's panel, more on the centre panel). What remained on the F/E panel were essentially the electrical and fuel system controls and indicators. And yes, the F/O seat rails were extended to permit that occupant to slide back and deal with items on the F/E panel. I don't recall them going really far back so perhaps they were shortened when the two-pilot operation was nixed, however they did go back quite a way and many a S/O suffered bruises when, without thinking, an F/O slid back suddenly (part of an F/O's checkout was to warn the S/O when coming back!).
AC had used licensed F/E's on its Super Connie fleet but they were never employed as such on the DC-8. Some did obtain pilot licences and became DC-8 S/O's but as PILOT officers, albeit doing what was essentially the job of an F/E in flight.
AC trained the S/O's to full F/O standard, including flight training, but since no line indoctrination was done it was able to block the award of a licence endorsement, presumably fearing that young pilots would just use the job to get endorsed and go elsewhere to earn a proper living flying DC-8's, initial pay rates at AC then (as now) were barely subsistence wages. Although the regulator (called the MOT then I think) refused two-pilot ops, things were pretty chummy in those days between the government owned airline and the government Ministry of Transport. S/O's were allowed to spell front seat pilots during flight (many sat in the captain's seat on eastbounds while he enjoyed his dinner in the first class lounge, and spent many hours in the sun on westbounds). There was no prohibition on occupying a front seat during takeoff and landing but SO's were prohibited from actually accomplishing those tasks. Was quite common for an S/O to fly up to the OM on approach and rumour has it, even past that point. S/O's were integrated into a three PILOT crew although, as the story goes, they were primarily there a sexual advisors to the captain. :-)
Looking back it was a bit of a challenge operating airplanes from so many series, -41,-43,-53,-54,-61,-63,-73 with a variety of engines running steam driven gauges - all in a small fleet of just 40.
Finally, I had heard that Canadian Pacific ordered its first DC-8's in the odd cockpit configuration but they were either not delivered that way or were subsequently modified
to the standard Douglas cockpit.
A neat bit of nostalgia for me, it's coming up on 52 years since I did my first (of three) conversion courses on the DC-8