I've relocated the battery in my Alfetta Sedan to the trunk and I've yet to reconnect the positive power line for the stereo that used to be connected to the positive battery terminal.
The only easy positive lead source I see is the starter solenoid mounted on the firewall. Is it OK to connect the stereo to it? If not, where should this be connected? When i got the car, the stereo was wired to the battery with a well-integrated on/off power switch for it on the dash to the left of the steering wheel.
Usually you'd want to run it to the fusebox, yes. Depending on the fusebox type there may be open terminals, or you can wire it in to the input side of one of the existing fuses. You should add an inline fuse to your new power line as close as possible to the fusebox for safety.
Usually you want to hook up the radio to the switched power portion of the fusebox so that you wouldn't need that on-off switch. If the radio has memory you'd wire the memory to +12 hot and the main power to +12 switched.