that really is the wrong psu for that motherboard. it is a atx-compliant psu, which means that some of the wires in that 24pin harness allow the pc to tell the psu when it is turned on and when it is turned off. you can try to hook it up, but you might have issues with starting up and shutting down, or you might need to short the sense pin to get the psu to believe the pc is there..
you really only need 10 ga wire for the psu, unless you plan on running other devices off the same wire. the fuse rating is correct for the same situation(powering only the psu).
just be sure to use the same wire, or larger for the vehicle chassis connection.
using a larger 8ga wire won't hurt anything, the biggest difference is that it allows for room to add other devices, and costs more then 10ga wire.
for the ignition line, you really only need 18gauge wire, and a fuse is usually not needed, but won't hurt anything to install. if you do use a fuse, use something around 1 amp. that line will not draw any more then about 0.5 amps at the absolute most.
as for wiring it, check the manual for the psu-- that should give you all the details of it.