We've all seen the old Charles Atlas ad where the skinny guy gets sand
kicked in has face and becomes a "man" after taking the muscle
training course:

http://www.steveconley.com/pages/atlas.htm

I have a distinct recollection that I saw a comic in the late
sixties/early seventies where Mac, the skinny guy on the beach, is
shown or at least implied to be smoking marijuana, and the bully
admonishes him for it.

Mac, presumably after conquering the munchies, then follows the
training course and conquers the bully as in the normal ad.

I'd like a jpg/gif/scan of that variant of the advertisement.

This sounds very familiar. I think it came from one of Gilbert
Shelton's "Fabulous Furry Freak Brothers" comix.

I found a reference to a "Charles Acid" Charles Atlas Ad Parody at
http://pers-www.wlv.ac.uk/~fa1871/LondonOz.html in the Nov/Dec 1970
edition of London OZ.

No picture, but might that be it?

Hmmm... I don't think so, because I don't believe what I saw was a
parody... it most likely was in a batch of old Archie comics I picked
up at a garage sale as a kid.  I don't remember reading the London OZ
:-)