proper ga wire to blaupunkt tha1350

i currently have a 1993 mercury cougar with a 150 amp fuse going to a (1) 4 ga to (4) 8 ga distribution block. i am powering the front speakers with a kenwood KAC-629s (60w RMS x 2 @ 4 ohms) and will soon be getting a blaupunkt tha1350 to power a 12" infinity kappa perfect, and have a couple questions. i have read recently that using too narrow a diameter power cable can cause overheating in the cable, possibly causing a fire. is this true? and will running a second 8 ga wire to the sub amp alleviate this problem? i know that may sound kind of unproffesional but i didnt anticipate having an amplifier with that kind of power and dropped 30 bones on the distribution block without thinking of future expandibility. one last question; is a 150 amp fuse too much allowable current in case of a short? the total of the fuses on the amplifiers equals 115 amps (not sure if that really matters too much, but hey i figure if anybody knows it'd be you all)

do it right, get a different block. 115 amps needs 4 guage. sell other block to friend or keep for future installations

alright i got a diff. block and now am having trouble getting the 4 ga wire through the firewall... i dont have anough experience to want to drill through it, so im taking it to a shop for that...