The roadster has a metal seat frame built into the floor. The bottom springs sit down into the roadster seat pan. The back of a roadster seat is seperate from the bottom and mounts to the package tray
A coupe has a more flat seat area and the bottom of the seat is flat wood. The coupe bottom cushion can be removed and there is still a wooden seat frame base, with a padded back attached, and seat frame sides.
I believe the coupe seat is narrower than a roadster seat. Does it fit drop in same? No. Can it be made to fit? Yeah. Since you say your roadster has a Van seat in it now, I'm assuming that you don't have the stock seat pan in your roadster, so probably the coupe seat will work fine for you.