Exterior.As mentioned, body line wise, ’70-’78 were pretty similar. The creases, roof, fender, and hood lines were all the same. Exterior differences were the
bumpersof course, taillights changed at the introduction of the 260. The front lower valence on the late 260 and all 280’s is lower,
(to accommodate the larger radiator and open up the radiator opening to compensate for the larger bumper), and the late 260 and all 280’s also have the front turn signals in the grill just under the hood instead of the below the bumper in the valence corners as on the 240 and early 260. The very
early 240had the interior vent outlets on the bottom of the rear hatch, late 240 and all subsequent 260 and 280 cars that interior outlet vent was on the "C" pillar behind the badge just behind the quarter light.
Interior. In general terms there were two distinct interior designs. The 240 was one, and then the 260/280 was the other. The 240 didn’t have to many interior changes through its production. The 260 interior was totally revamped. The 280 retained the 260 interior. In ’76, the shoulder belt mount point went from the “C” pillar to the strut tower,
(seat belt doesn’t dig into your neck as much on the ‘76+ cars). Also, in ’76, the
AMP gaugebecame the Volt gauge with a fiber optic charge light. In ’77 the only significant interior changes were the font style of the gaugesand the doors received a total redesign internally, which relocated the
window crankand the door lock switch. The door changes in ’77,
(which carried over to the ’78), made for much heavier doors, but the window regulatorswere much smoother and the actual door striker design was totally different than the earlier ’70-’76 doors. Doors for the ’77-’78 will not interchange with the earlier doors due to the different striker design. In ’78, the speedo received KMH in small blue print. Either in ’77 or ’78, under dash foot well lighting was also added. In ’77, the Z received larger capacity fuel tank which encroached on the spare tire well necessitating the space saver spare,
(the 240, 260 and ’75-76 280 received a full size spare tire), and also the ’77-78 rear hatch area now has a raised false floor which was to accommodate the larger fuel tank and that little deflated spare. Depending on which manufacturing plant the ‘77/78 was manufactured in dictated how the that false floor in the hatch area finished out to the rear. Some were flat level all the way back, others kicked up at 45 degrees to the hatch.
Structural. There were subtle structural differences as well. The late 260 and 280, the trans tunnel was widened substantially at the bottom. For some guys doing V-8 conversions, that is a big plus for exhaust routing. The T/C rod mounts were beefier as well as portions of the frame rails on the late 260 and 280 vs the 240 shell. As mentioned above, the late 260 and 280 had a larger
radiatoropening and as such, the lower core support dropped.
Suspension. Functionally and in basic design, they were all similar. The 280 strut tubes were a little thicker, the 280 had a little more caster, it has been said that the 280 rear control arms are a little heaver gauge material. The front cross memberof the 280 is a little beefier as well. Brakes were the same, though the later 280 had anchored dual piston wheel cylinders in the drums vs the earlier sliding single piston wheel cylinder. The 260 received slower
steering rackand pinion and the pinion housing was cast iron vs the 240’s aluminum.
(The rack and pinion may have been and early to late 260 change, someone else here would know for sure).
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