If the stock paint is still good (not cracked or glazed ) Keep it on . Just think of if as a super tough base coat. Sand off the black smooth up the stock stuff and prime over it. Going down to bare metal can cause a whole bunch of other problems. I sanded through on a couple spots on my charger Now i cant get my primer to cure in those areas . Also It will require more body work to get it straight again. We just did a friends 62 impala. It was prety straight to begin with he sanded it to bare metal. After the first coat of primer all the highs and lows showed up. After 2 1/2 gallons of primer and a whole lot of sanding we got it straight again. If your intent on going to bare metal I know people that have used ground up walnut or almond shells as a blasting media. Its suposed to be ok on the sheet metal but I havent used this myself. These are plentiful here in as there are plenty of nut processing plants in my area.
Good luck on the project Dawger