That bass riff is really a standard tumbao (the Cuban term for a repeated figure on hand drums - tumbadoras - and other bass instruments, marimbulas, botijas, and string basses). It can also be heard on the famous LP "Puente In Percussion" on which Mongo and Willie Bobo participated.
McFarland's presence as an arranger on "Soul Sauce" is not confirmed, although it is probable he wrote some arrangements, especially since one of his tunes was recorded. McFarland and Tjader recorded vibes duos for Verve that were not issued; On "Solar Heat" they play together on two sets of vibes, on two tunes.
McFarland had a far better knowledge of Cuban and Brazilian music than most of his contemporaries, an innate feeling for the music. Listen to "Sting of the Bee", which could have been written by a Brazilian.
He could have heard the Mongo and Puente LPs just as well. Tjader used that tumbao on a track on a Verve LP, I just can't recall right now on which.
What struck me about the soundtrack for "13" is that he developped the whole score just from that one theme - really outstanding.