If you've got good adhesion with what's already on it and no rust issues, then unless the existing paint is "too thick" I'd probably sand it all over, lay down a coat of good epoxy primer as a sealer, then go with paint.
Which type/brand/line are you planning to use? Base coat/clear coat? Single stage? etc.
if you can swing it I highly recommend Kevin Tetz's "Paintucation" DVD's -- terrific information. Get them "various places" and/or at paintucation.com
Also he has a forum up at Kevin Tetz's Paintucation Forum
that's a great source of help.
Unless you're prepared to spend 'big bucks' on building a booth you're just not going to be able to keep all the dust and trash out of the paint, so just be ready for that ahead of time and "be prepared" to correct the spots afterwards. Are you planning to color/wet sand it after painting and then buff it out?
I can tell you that when I painted my 69 (also black) I shot it in my garage, no plastic or nothing --- I bought 4 cheapo box fans and built a wooden framework around them so they'd fit underneath my garage door. I arranged them with 3 blowing in, 1 blowing out - the idea that the net "positive pressure" inside would tend to force dust and crap -out- intead of drawing it in. I put hvac filters over the fans to filter the air they moved. It worked "fair" - meaning I guess they helped a little but I still had a lot of trash in the clear I had to deal with afterwards.
Honestly? I really couldn't "prove" they helped any at all.
I've also heard of people just painting outside in the open air and getting pretty good results, and also others putting up one of those "temporary" garages that's really just a big tent over a metal frame. With the latter you have to be careful about the material used in the awning, some of it makes a lot of lint.
If I had it to do again, and didn't have to worry about neighbors, I'd probably try the open air route, provided I could get a calm day with temps in the 75-85 degree range.