2 gauge is fine for the path from battery to starter, and from engine block (ground) back to the battery. With a firewall mounted battery, you might be ok with 4 gauge.
6 gauge is more than adequate for that size alternator; 8 might be adequate, depending on length.
The gauge measurement doesn't change between a/c cable & welding cable.
The advantage of welding cable is that it's typically a lot more flexible; the (potential) downside is that the insulation is thicker, so it may affect the ability to get it through existing bulkhead passthroughs if they are already near full.
Charlie