I usually don't post colorized pictures as I think it takes away something from the content. I'm making an exception... and at least they got the engine colors right.
Trenton engine plant, big block Mopars. 4 bolt valve covers makes this 1963 or earlier. Note convertors are already installed.
From the comments with this picture, this was from someone that worked in the plant from 1972-1980:
"This is the engine Hot Engine Test line. (Engine Carousel) Every engine was started and tested. The workers hooked up the exhaust hoses, to the exhaust manifolds water lines. They had a unique natural gas system the workers used to place on the intake manifolds to start the engines, since no carburetors were not installed at Trenton only at the final assembly plants."