The 3800 series II block can handle alot more than the 2.8/3.1/3.4. You can use an older non-electronic tranny or manual, by simply fooling it intothinking it's in Drive all the time(very easy). The older 2.8 700r4 will work, as will the newer 3.8 4L60e(from a Camaro), but I'm not sure how you'd control the 4L60e.
People are running over 400hp with the L67, with ALOT of torque, so it's definately a capable engine.
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-T
'87 Lesabre T-type w/ L67 14.447@95.08 (2.299 60ft.)
'86 Regal Limited w/ '80 Buick 350 (old unfinished project)
'88 Grand Am w/Iron Duke and 5-speed (winter beater)
The Lesabre T-type Page