Just found this thread and I believe I am qualified to answer :P
Plain and simple what are you looking for
S1 L67 in a Riviera is better to get the car moving because of the lower torque peak but it runs out of breath at about 85 MPH (stop light racer)
S2 L67 in a Riviera is more midrange power with a higher RPM torque peak. But it still has breath until triple digit speeds. (1/4 runner)
question at the end is how much are you going to mod it.
One point I want to make is the Riviera is the slowest of the L67 cars due to its weight. Keep that in mind while you are shopping.
2001 Bonneville SSEI.
Engine: GEN V M90, ZZP 3.4 MPS, Intense S1X Cam, LS1 TB, SS autochrome headers, ZZP UD WP, DHP Powertuner, 3.29 Gears & DIY Shift kit