the 391 had one in a 68 3/4 ton don’t know what made it any deferent then the 390 looked the same but was told all the 390s in the trucks was 391s the 390 was a car motor the spark plugs was not the same
361 and 390 were truck specific, mostly to the heavier duty trucks. They are virtually the same, except sometimes the block and crank are beefed up. They are interchangible with the car motors, although the extra beef is not really helpful in performance applications. Light duty trucks still got 360s and 390s.
I owned both a 1976 360 and a 390 F150 and F250 super cab trucks. The 360 was not a performer at all. A very poor setup. The 390 was a factory 4 barrel and what power. I think the odd displacement specs were found in the larger trucks 350 and up.
