I've been placing orders for the remainder of the stereo system. The rear speakers came in yesterday. I went with Infinity Reference marine speakers for the front and rear, because since I'll be driving around most of the time topless, they won't get ruined if I get caught in a sudden rain storm.
Fronts are 6-1/2", rears are 6 x 9's. I'm going to build individual speaker enclosures, and coat them in black Monstaliner.