Frankly, I think you can get away with either configuration, although it appears, based on my research, that shoulder harnesses were usually a restoration item, and were not installed on the original aircraft. This comes from a SWAG though, based on looking at numerous photos of pilots or co-pilots in the cockpits, and a lack of visible shoulder harnesses, yet it's a 50-50 mix on the restored Warbirds.. "Liberty Belle" had them, "Texas Raiders" doesn't, "909" has them, none are installed in "Sentimental Journey", etc..
Here is a co-pilot seat without.
Pilot seat
The co-pilot picture also shows the quilted sound-proofing that was installed in the factory, but often removed after it went into combat, as it was a PITA to reinstall after repairs were made or battle-damaged..
Waist looking forward.
There should also be a firing step about four feet long below and centered on the guns, and that are about eight inches higher than the floor, which should be plywood with non-skid (on late-model Fs and all Gs) surfacing and the floor itself shoud be plywood (rather than the custom-made aluminum one shown here for location purposes only) and it also has a non-skid surface applied as well...
The kinda gruesome but practical purpose of the non-skid was to give better footing on a blood-covered floor..(we're talking about the reality of combat here). Same thing was applied to the floor of the radio compartment in places..