I just looked at a lady's '91 Rodeo and it says in the engine compartment it has a 3.1 litre...any chance this is the same general motors 3.1 in the older buick centuries and Luminas? I don't think it is but I thought I'd ask.
Your 3.1 is a cast-iron-headed version of the 3.1's that came in FWD GM cars. Biggest difference is the pistons, which have less dish in them to account for the larger size of the cast-iron heads' combustion chambers.
The FWD blocks typically used aluminum heads which have smaller chambers, thus bigger dish in the pistons to keep the compression ratio from being too high.
Crank and rods are the same, and if the Rodeo is 4WD the starter is located on the same side as the Front-Drivers. If 2WD, starter is on the pssgr's side same as you'd find in an S-10 or Camaro equipped with the 60-deg V6.
Other differences as well, this is a good site for 60-deg V6 info: