Two other thoughts/questions...how much different does the gbody Monte Carlo interior look from an olds? I’m considering a bucket seat , console shift and steering column swap from a 1986 Monte Carlo into my 80 cutlass .i would prefer all Oldsmobile but this stuff is local to me and might be an option
1) the seats are no big deal. However, they will have vinyl backs instead of that plastic she'll your 1980 would've come with. Appearance is different than the olds which, IIRC, is the only gbody to keep using the plastic back shell through the 80s. The others generally stopped around 1981 give or take a year or so.
2) that floor shifter may have a slight issue. 1986 monte should have a 4spd shifter for the OD. Your 1980 would've used a 3spd shifter due to no overdrive. If you did, or plan, a drivetrain swap that may not be an issue, and, cam be worked around anyways, but it is a difference you should be aware of