When I bought my 70 R/T, the title read that it was a 71. As I purchased it from the original owner, I got the straight skinny first hand.
The story on my car was that it was built in Sep69 with virtually no options in an undesirable color. It simply didn't sell. It sat on the dealer's lot from Sep69 until it was sold at a substantial discount in Jan71. A clerical error (yes, they happened back then also) showed the car as a 71 on the title and on each of the license registration cards. When I purchased the car in 1978, I "righted the wrong" with the DMV (which isn't always easy from folks who claim they never make mistakes), and enjoyed the car.
There's no sin with installing 71 parts on a 70 unless it's being done for unsavory reasons (i.e., fraud).