The first thing is to clarify that you have a true bench seat: a one piece seat that handles both the driver and passenger. Many (most?) people have the split bench (60/40). (I did have a wagon with a true bench seat and it was not common.)
If you have a true bench seat, you will have to throw it away and start over. Go to the junkyard and find a set of bucket or split bench seats to work with. (Split bench seats can be modified to remove the middle section and easier and cheaper to find in the junkyards.) For ease of installation, try to find a B-/D-body seat (Caprice/Impala, Roadmaster, Custom Cruiser, Fleetwood). Bonneville seats are very comfortable and can be adapted to work too. There are many threads on the other forum that you can give you info about this. (Just about any medium- or full-size car seat can be adapted with some work.)
Cars with true bench seats also may not have the inside pair of mounting studs needed for separate seats. You can secure the seat with bolts, but make sure to make a plate (not just a large washer) to bolt through so that seat does not pull through floor in an accident.