Lakewood makes a nice aluminum bell. It fits the big 308 bearing front retainer and also has spacers included for other sizes.
Lakewood LK6000 Lakewood Bellhousing - Big Block Mopar
I dont think there were any OEM big block bells in aluminum. Someone correct me if im wrong on this. If you want factory iron, search for PN 2892626. Im sure there are other compatible factory bells.