E58 engines were all manufactured with a special care process vs the normal engine assembly process
E58 blocks are a high nickle cast iron block, full groove mains bearing with special inspection process along with tighter tolerancing with the crank, rods and piston, Rods were magnafluxed and shot peened for strength and durability Ad double roller timing chain, lubrite treated camshaft, windage tray and heavy chrome piston rigs completed the short block. Heads were hot alkali cleaned for more effective cooling passages. Valve springs were hot pressed, exhaust valves were a mnemonic ally with chrome flashed exhaust stems, silichrome-1 high temp steel intake valves, viton valve seals and HD rocker arms. As part of E58 the car would have the HD cooling assy consisting of an AC water pump with the large bearing, 26 inch radiator, a 20 inch fan with a thermostatic clutch

As far as black valve covers they came with the orange silicon valve cover gaskets. E58 Cop cars were supposed get painted valve covers as did the 1979 Chrysler 300 1979. Little Red Express got chrome valve covers. Super Coupes, Kit cars have been seen with black valve covers but never say never as far as that goes. Cordoba's & Magnum's have been seen with them as well. All E58s were assembled on a special line but the black valve covered engines were said to have been assembled at Chrysler Marine. I have no proof other than the research and interviews that I did back in the late 80's That is what I was told by the people that worked at Chrysler in those times.

Btw a well tuned E58 360 is no slug. Properly & well tuned are key words. Just a cam change alone woke them up big time. Up the compression to 9.0:1 with 202 heads as the 68 340 cam and it will beat up your 340's built the same way all day every day.