Stock is a good way to go, but that's my liking. Used are available and most new parts are available from an Impala Vendor. But even going Used, I recommend some of the parts be new. A combination of both is your best bet and pretty easy to install. Other than the new conversion kits, I can't really help on any other non-original application.
-Parting Out over 75 '58 to '73 Full Size Chevy Cars-
-National Impala Association
-Vintage Chevrolet Club of America