[SS427]

I can answer most of these Joseph and others will also chime in on the rest.
-did all 70 chevelles have the two piece brake rotors
No, only the early cars had the two piece
-did they all have the plastic inner fenders
No, some had both, some had both steel and some had one of each.
-was 70 the only year for the flat bottom windsheild washer container and the fluid level monitor option.
I can only answer for 1970 and they were the flat bottom. The later verson with the same part number had the angled bottom.
-did all chevelle big block cars have the adjustable lower steering shaft coupler
Yes
-were the dash vent louvers smooth black not textured only in 70.
Yes
-was there two different master cylinders 309 dual clip and single clip
Only the single bail "409" cylinder was used on the disc brake cars.
-did only big blocks have the oval lower control arm bushings
The SS Chevelles used only the round bushing.
-original exhaust tips welded on with no part numbers
Yes
-back up lights section in tail lights raised not flat like replacements.
Only on early cars.
-location of white plastic hood pin cables
Varies by plant
-reinforcement welded to upper left training arm.
On all 12 bolts as far as I know.
- Dual fuel lines
Only on late cars.

Rick