A 64-67 rear seat is the same. A 69 is not. You need buckets from a 69-72. 69 is slightly different than a 70-72 bucket, but most would never know the difference.
Years ago, I put a '69 GTO rear seat in my '66 hardtop... It worked and fit well... But I doubt that you couyld have '69 covers put on...
Also, while "correct" buckets will be 69-70, 71/72 can still work with '69 upholstery, but you will need to cut the foam for the side bolster attachment... 66-68 seats will work and look simillar to '69... Might need 68+ foam to use '69 covers.
the big difference is 66 has not bacl lock, 67-68 have a side-mounted release button for the back lock and 69-72 have a center-mounted back release button...
64-65 Cutlass seats will function, but the style is completely different...