I've got a '67 Cuda convertible that I was planning on installing a nice subwoofer in the trunk, and then some smaller speakers probably under the seats in front and then a couple more under the dash. I know there's not much space, but I'm sure nowadays there are a lot of speaker options, size-wise.
Also, remember that with the top either up or down, you're going to get a lot of sound coming out from behind the back seat.....(bass, that is..) if you've got a nice speaker box in the trunk, of course, out of the way of the convertible top assy going up and down. The speakers will need to have their own boxes, to produce a decent bass sound.