Are the differences the exterior sheet metal,dash and bumpers and such?? All the chassis parts are the same right? What about body mounts, gas tanks.

You've got it basically. Suspension, steering, brakes, and most body mounts are the same. Gas tanks may differ

Absolutely IDENTICAL .. No! Vertually nothing on my '69 is identical to any other year. Most of the parts will interchange though.

The MAJOR similarities and interchangeability is related to chassis/drivetrain. After that, only a few things are the same.
Just a few examples, both ways.
The 68 steering column is a ONE YEAR only column. The 68 cars (across the board) were the last to have the ign swithch in the dash, the 69-later cars got the switch moved to the steering column which included an interlock.
Things such as seats, door panels and some dash components will interchange for 68-69. Most of the sheetmetal for 68-69 are interchangeable. The 68-69 tail lights are totally different, BUT, the complete tail light housings can be swapped (have 69 tail lights on a 68). Front bumpers will interchange, but the park lights are in different positions in the bumpers. Hoods will interchange-----------BUT REQUIRE THE HINGES ALSO BE SWAPPED!
The 68 cars had the air cond compressor (right) and alternator (left) on the "correct" side. They got switched in 69 (requireing totally different wiring harnesses under the hood). Thus, ZERO interchangeability in engine accessory brackets!!!!!!!! But chassis and drivetrain parts are a direct swap.
There were TWO distinctly different REAR bumpers for 68 ONLY. Malibu bumper (holes for backup lights) and 300 bumper (no holes, backup lights were incorporated into the tail light lenses). Thus, different wiring for the rear in 68.
Gas tanks changed for 70, and again in mid-70 for the evaporative system.

The 70-72 are completely different in regards to sheet metal, roof, hood, and dash.
The 70 front fenders, grill, radiator support and front bumper are one year for 70. The reason is because of lights. The 70 had dual headlights and the side marker lights were in the fenders, thus, slits in the sides of 70 front fenders. The 71-2 had the side marker lights incorporated into the park lights/bezel. No holes in the 71-2 bumper for park lights, and no holes in 71-2 fenders for side markers. The 71-2 radiator support is the same (for single headlights), but the 71-2 grills, headlight frames and park lights/bezels are different. Rear bumpers are the same. MOST sheetmetal for 70-72 is the same (except front fenders).
The 68-72 trunk lids interchange.
Much of the 68-72 wagons and El Caminos shared the same sheetmetal/tailgates (70-72 models kept the body side lines of the 68-69 models).
The 69-72 steering columns interchange.
The flat, cowl or Cowl Induction hoods are totally interchangeable for ANY 70-72 model (2-4dr, wagon, Elky, doesn't matter).
The air cond components, especially the condensor, are one year only for the 68.
The INNER front fenders are the same (and battery tray) for 68-72. BUT, the 68 RIGHT inner fender has extra holes for factory air (if you install a 72 right inner fender on a 68 with factory air, the holes have to be added).
The 68-72 10 and 12bolt rears, suspension, attachments, etc, are all the same. A 72 12bolt rear, boxed trailing arms and sway bar is a 100% swap into a 68 with a 10bolt rear.
These are just a FEW examples of what will and will not swap. I don't care to continue on forever pointing out the differences, but you can get the idea from the above.
The actual frames are the same for 68-72, except the cross member at the rear of the frame rails may be slightly different between 68-9 and 70-2 frames (I have never mad an actual side-by-side comparrison.
ALLLLLLLLLLLLLLLLLLLLLLLL 68-72 frames (2-4dr, wagon, Elky, conv) have the two holes punched in them for attachment of the clutch Z-bar bracket, REGARDLESS if the car was built as a manual or auto tranny car.

Hey DZ Auto - how about some useful information! (joke!) Wow... nice post. You've done this before!

Hey DZ Auto - how about some useful information! (joke!) Wow... nice post. You've done this before!

In my family, we've had 68, 70, 71, 72 (not had a 69). This includes a 72 wagon, so I'm familiar with the difference between wagon/Elky and 2-4dr sheetmetal.