Personally, I'd throw on an M-16 grip, 10 round tube, take off the ghost ring and stock, use macro instead of hardline, run the 3.5oz. tank vertically, remove the pump return spring, and get some detent rings.
That's just me, that was also how I had my old Phantom set up, minus the gauged valve, and I never bothered with a harness system. I just put my tubes in my squegee pocket.
Anyways, all that gun really needs is a set of detent rings and you're ready to rock. Nice find.