Charging issue

[Rattletrap]

Mustang with battery in trunk show 12.5v while idling, 13.3ish at alternator.
Motor is a 6.0 with EWP and UD Crank pulley with truck accessories.
And I am making the assumption that even when driving around at 3k there is not enough juice getting back there since it wont start if I cut it off.
Battery is grounded to frame @ rear, then ground from chassis up front to block.
Positive wire from battery is huge stereo cable from rear to front where I have a bulkhead and the alt wire is connected to it. and So it all the other stuff that needs power.
Starter wire runs from/to this bulkhead too.

This sounds like what most do with all the mustangs and relocated batteris, well they run it from/to the starter 'noid but the same concept.

I have run a ground wire from the alternator casing to the chassis and no change at battery.

Looking for suggestions. They have ranged from:
-I need a 1/0 wire for ground from battery to the back of head/block
-maybe the UD pulley needs an OD pulley since the battery is moved
-maybe battery needs replacing-wont take charge-its an 09 Red top

I do not like relocated batteries-this is the major reason, would rather run a small LW battery up front...