Bear in mind that 3.7 Roadsters (they were never known as S4) adopted the factory 5 link coil sprung rear from April 2018. They still benefit from the front SSL RS kit but dual rate coilovers to match are offered for the rear as well. SSL offer their own retrofit coil spring 5 link version but it's quite expensive although provides similar very positive axle location lacking in all other Roadsters and so important in these powerful Morgan's.
Suggest you try both, the Roadster S1 seems to be the jewel of the 3 litre ones in terms of power and transmission. The 3.7 has a real Jekyll and Hyde character though with quiet very flexible smooth engine until you reveal the monster when the Ti-VCT kicks in
