I got them from these guys. I think they are made by procomp. Anyway..Mine is a 340 but they make 'em for the big un's also,
http://cgi.ebay.com/ebaymotors/NEW-BB-MOPAR-383-400-440-Fab-Aluminum-Valve-Covers-/380231618149?cmd=ViewItem&pt=Motors_Car_Truck_Parts_Accessories&hash=item5887920e65