There wasn't one year that they switched over - it was a gradual process over the 70s, 80s and 90s. Some non-metallics were single stage well into the 90s. Non-metallic black (040) was single stage from the early 50s until about 2 or 3 years ago. If you are talking 040 black for the top I would certainly go single stage. There's fewer components to buy, it's easy to put on, and I think it lasts longer, assuming you buy a high quality urethane paint. You don't really even need to be very good with the spray gun if you're willing work - wet sanding and buffing. You can get a perfectly flat, incredibly glossy result if you're willing to put the labor into it. That the great thing about single stage urethane - anybody can do it if you're willing to work. If it was the whole car the labor could be an issue but it shouldn't be that big a deal for just the top.