Finished building the bench seat using the frame I got from TKCR, which as I understand it was a "deluxe" seat for 77-79. I used replacement springs along the sides from usa1industries.com, new deluxe foam and cover from classicindustries.com.
As a reminder, I'm building this to use instead of the "cheap seat" with springs and a thin foam pad I'd gotten from a low-end 79 model. Is the new deluxe full foam seat more comfortable - well hell yeah! Of course I'd say that anyway after driving a couple thousand miles meeting TKCR to swap frames, cleaning up a second frame and painting, replacing broken springs, etc.
Seriously though, it just "feels" better not hearing the springs skreak whenever you climb in or out, or just moving around on the bench. This foam is much more firm than the other seat and just sitting in my garage it feels like a little better back support. Haven't taken a long drive with either seat so my judgement of this is hardly a basis to change your own bench
Not sorry I went to the bother though!