I wanted to add some comfort to this old cruiser so I purchased a set of leather seats out of a 1996 Bonneville for $300. The seats were blue and matched my stock blue interior. Due to the fact the Bonneville rear seat back is not a perfect fit (too narrow),I decided to do the job right and ensure the rear seats fit as should be from the factory by using the stock caprice rear seat frame with the Bonneville seat foam and leather upholstry. Here's a pic of the new rear Bonneville seat back looked like before I started doing surgury.

Here's what it looked like after I stripped the foam away from the Bonneville frame.
Here's what it looked like after I cut out the center console frame bracket.
Here's another view of the bracket. Note: I brought the head rest brackets as well. No cutting of the foam and glueing with this job as is suggested by others who have previously performed this modification. Unfortunately I did forget to take a picture of the Bonneville foam. I must admit, it took me at least 2 hours to seperate the foam from the bonneville frame.
Here's what it the factory 9c1/Caprice seat frame looked like after I welded in the console bracket and the headrest brackets.
And here's what the final product looks like using the Bonneville foam and leather upholstry on my 9c1/Caprice frame. With some muscle and patience, it all fit like factory installation
I did the same thing to the rear seat pan area. I stripped the leather then the foam from the Bonneville frame. Using the Bonny foam and the Caprice seat frame I carefully installed the leather seat cover for that factory fit.

The only modification i decided to perform was adding some filler foam around the front edges of the seat foam. When you place the Bonny foam on the Caprice seat frame, you can clearly see what needs to be filled. Cutting the old Caprice foam was a piece of cake using a hacksaw. And here's the only tools you will need.