It'* either the Riviera or the Park Avenue that'* the slowest L67 powered vehicle.. My guess is Riviera though just because the G body chassis is HEAVY. I'm not sure of the weight on the Park Ave, but the late Riviera'* were nearly 4,000 pounds.
A friend of mine has a 98 Regal w/ leater and a bunch of options. He'* @ 106k now and the only thing he had to replace so far was the */c coupler. He'* been modding for over a year and currently runs 13.65 with a cai, 3.25" pulley, alky injection, 3" DP, cutout, tuning, and stock valvetrain/bottom end/tranny. Not to bad for not digging into the motor at all.