The Hackman movie you're thinking of is
The French Connection, I believe, Osso.
Just a point of reference, assuming the Freightliner in question was a full sized Class 8 road tractor, with a typical 50' closed-van trailer, the available load capacity would be in the neighborhood of 25 tons. Baled, in the fashion of hay, you could prolly get just about that much pot into the trailer, given the cubic volume of the typical trailer.