If it was my money, I would go with one of these:
(They are used but you may be able to find something new)
http://www.ebay.com/itm/76-77-78-Volare-Aspen-Coolant-Radiator-Overflow-Tank-Bottle-Mopar-F-Body-/360853313061?pt=Vintage_Car_Truck_Parts_Accessories&hash=item5404889625&vxp=mtr...