[68Gold/white]

My interpretation of the words, "light duty", "medium duty", and "heavy duty" are, and always have been...:
Light duty always refers to 1/2, 3/4 and 1 ton pickup trucks. (C-10, C-20, C-30)
Medium duty refers to C-40 and above. I do not know where, what's considered "heavy" duty begins, maybe above C-70???

A medium duty engine in a light duty pickup, doesn't sound proper. Even back then emissions specs were made different for the weight ratings of these trucks.

I'm guessing GM's use of "medium" in that engine description is unique to the light duty engines...

In reference to the original post, I would guess one or more things making the engines different...valve sizes, different cams, possible better valve seats, maybe even a steel crank (I doubt it though...)

The text that factorystock gaave us, I know about...somewhat...the 350 medium duty (as in medium duty truck) engine came with a 2 Bbl Rochester carb. not the ordinary 2 bbl though. It was a special governed carb that would not allow over-revving... WHY??? some of those 350's turned 4000-4500 going down the highway (they had to to make the power needed to pull a heavy truck around). One slip of the throttle pedal and one of those engines would be done, if not governed...