If you have the full granny bench in the car (not a split bench), then you'll need the inner mounts on the floor to install either a split bench, or bucket seats. As others have already pointed out, you can either find a suitable parts donor car, and carefully remove the inner mounts, then position them and weld them to the floor in your car, or, the inner mounts can be purchased from several of the G body parts vendors, for a reasonable price. Then you go through the same procedure-position them on the floor in the proper place, and weld them in.
If the Cutlass bucket seats that you bought have no seat tracks on them, then you're going to need seat tracks as well. I've found through my own experience that there is a difference in the distance between the seat mounting bolt holes in the track, between bucket seat tracks, and split bench seat tracks of approximately 1". The seat mounting holes on a split bench track are approximately 1" wider apart than the seat mounting holes on a bucket seat track. You can fabricate adapter brackets to correct this, if you're so inclined. If not, then you need to find the correct bucket seat tracks to mount the Cutlass seats into your Monte. Your best bet would be to measure the distance between the seat mounting holes (side to side) on your Cutlass seats. It should be approximately 13 1/2". If you're looking at a bucket seat track, then that measurement should be the same. If you're looking at a split bench seat track, then that measurement will be approximately 14 1/2". Both seat tracks will fit the mounts on the floor exactlt the same-the only difference is in how the seat mounts to the track. As I said, I found this out through personal experience. I bought a power seat track from a Buick Regal for my Monte, figuring that there was no difference in the seat mounting points between split bench and bucket seats, and I found out otherwise. I had to fab up adapter brackets to get the bucket seat to fit on the split bench track.
Just a heads up for you. Good luck with your conversion.