If your amps are going to be mounted on the passenger side and that's where the batteries are located, I can't imagine that it'll be 16' long (unless the battery is in the back and the amps are near the bow). The location of your head unit and preamp have nothing to do with the power run to the amps. Just be concerned about the distance from the battery to the amps. As for wire gauge, it's really based on how many amps the amplifiers are pulling and the wire length. The following link will give you a good guide based on RMS power. From the sound of it, you'll probably want to go with at least 4 gauge. Hope this helps.
http://www.the12volt.com/info/recwirsz.asp