I did exactly that... during 1977-78, I converted my 72 318 rally into a 70 body and T/A... specifically for racing rules/legality (SCCA Solo II... racing against Camaro SS and Z28s, FBird TransAms, Boss 302s, Mach 1s, AMX, etc) ... and have totally enjoyed the results... racing it, driving it, showing it, etc... not having to worry about the mega-buck value of an original... so many of them just collect dust and never driven to use their power or handling potential.... beauty is great, but driving it is better!
As Mopar Action 2006 August feature article on my car said... "Mitch's car didn't leave the factory with the T/A stripes, but he's more than earned them!" I've also got a few hundred awards earned from racing it, as well as many show awards.
IF I ever sell it, I won't attempt to sell it as a 70, but the truth will be fully exposed. Anywhere I go I get 2-thumbs up all the time!
Many cars are cloned and/or changed for model years for the owner's personal reasons... Shelby Mustangs, Hemi Darts/BCudas, RS/Z28, etc.. even plenty of foreign cars.