The 68-72 models were very similar in their outward appearance...though there were certainly a lot of small cosmetic and trim differences. Here is some info and a few highlights...
http://www.novaresource.org/history.htm
http://en.wikipedia.org/wiki/Chevy_Nova
1968 was the last year they were called a Chevy II...first year for the new body style...also last year for the key in the dash...first year for the big block option...last year for the 327 engine...first year for the 350 engine...first year the TH400 was available...
1969 was first year TH400 was an RPO behind the L78 motor...last year the M22 tranny was an option in a Nova...first year for the locking steering wheel/key in the column...power disc brakes were standard with SS option...first year for front fender louvers...first year TH350 tranny was available...last year for redline tires
1970 was the last year for the big block option...first year the F41 rear sway bar was available...both big block motors had a low profile intake and were actually 402CID...only year for the COPO ordered Yenko Deuce...last year for high compression...last year for the 4cyl. engine.
1971 was a transition year...low compression, no big block, mild rear gears...other than the absence of the fender louvers the car looked the same in most respects...first year for the Rallye Nova option.
1972 Novas were still cosmetically the same with a few changes...the Skyroof option was introduced along with the high back bucket seats...1972 was the only year the SS option was part of the VIN...it was indicated by the letter "K"...last year before the impact bumpers made their debut.
The links at the top have a lot more details... hope this helps.
wilma