Agree with what Rob says above, lack of a height control valve should have no bearing on the rear ride height with only a driver and one passenger in the car and no luggage in the trunk. Your rear springs are sagged or broken. The "go to" place in North America for rear springs for a Shadow is Coil Spring Specialties in Kansas, they have the proper specs and they have made and supplied hundreds to shops and owners. There are also shims used under the springs to fine tune the ride height, this is all explained in the workshop manual in the rear suspension section.