Well I think the problem may lie with wheel offset.
As I understand it (but I may of course be incorrect) the 4/4 and Plus 4 share core bodies that are identical and just become the appropriate version when the engine, wings and right sized wheels are fitted.
With the Plus 4 wheel offset, rubbing doesn't occur but does with the 4/4 wheels. If Morgan had properly engineered their cars they would have different lock stop on the racks when fitted to different variants. Rover, for example, did just that when they put the larger Sportpack wheels on the late model Classic Minis. Lotus did the same with the Elan Sprint.
