Not sure of the mods necessary to put a C tank on a D but I'm sure someone has done it. I know thC tank is shorter so there will be a gap between the seat and tank that needs filled. And the C tank bolts on front and back so mods for mounting will be needed. I'm most concerned with the size of the tunnel om the underside and whether it is big enough. I likely Have a good C tank if you think you can handle the modifications.
Update : Maybe these will help. Looks like it takes some welding and tapping