[papajam]

Can't say much about how to mount the battery in the trunk except I've heard that some folks put the battery in a plastic marine battery holder to prevent acid from rotting out the floor.
Also, do NOT use welding cable for the battery connections. The insulation isn't designed for the harsh environments encountered in automotive applications.
As far as additional wiring, all you need is a 10 gauge minimum wire from the starter terminal where the battery cable is to the B+ terminal on the alternator.