most of this info comes from 60V6.

all 3.1 engines in W-bodies used aluminum heads, MPFI, flat tappet lifters, stamped steel 1.5:1 rockers and DIS. they are all Generation 2 60V6 engines.
they employ 28cc heart shaped(fast-burn) combustion chambers.
26cc dished pistons are used.
both the intake and exhaust valves are splayed.
the engine is internally balanced.
compression ratio is 8.8:1
1989 motors are rated for 135HP/180TQ, 1990 and later motors are rated for 140HP/180TQ. the difference is attributed to a better head casting.