The 542060 is the Pro series unit, 542013 is the High Performance unit. From the descriptions, the pro unit has 'higher bias torque' .... sounds like it has higher preload on the clutches. Either unit will fit. Chevy units and later 68-70 Buick units are different.
The HP unit is over $100 less, would be fine for a street car. I'd be hesitant to use the pro unit in wet or snowy conditions, it may not slip enough and cause the rear tires to slide out.
An Eaton unit is another option for 3.55+ gears. It uses conventional clutch discs vs the cone type clutch of the Auburn.
Best bet is to contact one of our Buick specialists.... Jim Monzaz has his own site at
www.jdrace.comHe can set you up with everything you need at a good price.