The type of coupe makes a difference. If it is a business type coupe it will have a different set up than if its a sedan type coupe. Some business coupes had no seat at all back there. Jump type seats were available though.
A sedan type coupe should be very similar to the 42 through 48 sedan coupe types. If the 42 through 48 sedan coupe spring box sets are different by some degree, it may be possible to modify them to make them work. If a person can find an auto trim man that has a lot of experience and some of the old tools used to repair these spring frames, it could make the seemingly impossible a reality. Where there is a will there is a way.