The mount for the trunk hinges that is under the upper rear body panel in coupes is not present in a Cabriolet, so something will have to be fabbed up to mount the body side of the hinge to. The Cabriolet lid will not have the holes for trunk mounting nor will it have the hole drilled for the trunk handle. There is also a sheetmetal panel inside the lower rear body panel on a trunk car that bolts in and has the catch for the trunk lid latch, so that will have to be added. The little wedges that center the rumble lid and keep it from rubbing the body when the body flexes are up at the top, in the drip rail, for the rumble and should be moved to the bottom for a trunk. If building a car from ground up it could be done reasonably easily, if trying to do it to a nice together car it's going to be more difficult.