I just finished installing a new cushion and backrest in my 59. I retained all the original 59 seat frame and replaced the coil springs with the rear seat from a early 90's DeVille and then reupholstered them. The foam Cadillac backrest is somewhat thinner than the old coils to provide a little more clearance with the steering wheel and got the fold down armrest as a benefit. The space behind the seat is not affected. It required a little cutting and welding, but was not a huge task. This might be something to consider if you are needing more space between the seat and steering wheel.
