W126 Body Dimensions.
Dimensions
Wheelbase Sedan (SWB): 115.6 in (2,936 mm)
Sedan (LWB): 120.9 in (3,071 mm)
Coupe: 112.2 in (2,850 mm) Length Sedan (SWB): 197.6 in (5,019 mm)
Sedan (LWB): 203.1 in (5,159 mm)
Coupe: 193.3 in (4,910 mm)
Width Sedan: 71.7 in (1,821 mm)
Coupe: 72.0 in (1,829 mm) Height Sedan: 56.7 in (1,440 mm)
Coupe: 55.0 in (1,397 mm)
W123 Body dimensions.
Dimensions
Wheelbase Saloon: 2,795 mm (110.0 in)
Coupé: 2,710 mm (106.7 in)
Limousine/ambulance/hearse: 3,425 mm (134.8 in) Length Saloon: 4,724 mm (186.0 in)
[2] (euro-spec bumpers)
4,640 mm (183 in) (Coupé)
Width Saloon: 1,784 mm (70.2 in)[2] Height Saloon: 1,435 mm (56.5 in)
[2] Curb weight Saloon: 1,625 kg (3,582.5 lb) (turbodiesel, automatic)
The Differentials will swap from a W126, 81 - 85 over to a W123.
The Gassers use a larger Flex Disk because of the larger Tri Flange. That would have to be changed if going into a Diesel.
W126 86 - 91 will not swap over, MB changed the Diff mounting.
The Trailing Arms are the same up to the 85 W126. w123 and w116 are the same.
Charlie