In this particular case, I have to agree with Steve. The body is a 70, but if it were not sold until 71, it might have been titled as a 71.
Something else that you have to remember about pro cars is that they didn't always follow the manufacturers guidelines. In the case of my 73 Superior Cadillac, it had 72 rear lights. The 73 rear lights on the Cadillac changed a little from 72 to 73 and Superior chose to keep the 72 light housings instead of going to the 73 style. I have had many an argument about my car being a 72 or a 73. If something from one year suited the conversion better, they carried it over to the next year.
Without going on forever, another example was the 59 Superior Pontiac. My father looked high and low to find the wheel covers for his 59 that matched the literature from 59 only to be told that they didn't exist in that style for 59. What happened was that in 58, Pontiac went to a smaller wheel on the passenger cars. Superior was worried about the smaller wheel not being able to carry the weight, so they kept the larger wheel from 57 up through at least 59. The wheel covers that he was looking for were actually 57 covers that were on the 59.