trying to help here, but most aftermarket alloy chev heads for big blocks have big valves and that wont go on the small 396 bore size, your better off turfing it and getting a 454 block or even the gm 502 or dart block they way the aussie dollar is, or use the block with some cast iron small port oval heads and a cam intake extractors they went ok, i just think putting alloy heads even if you found a pair to fit is a waste money could be put else where into the combination, it like rebuilding a 307 chev why do it?? unless its for originality you would be better of buying a dart shp block and make a 400= c.i engine the bigger the cubes more torque the more h.p you can make.
pete.