I'm not so sure about Robush silent bloc bushes. Their rubber seems much harder, less flexible than the originals. Hills Rubber Co. in Reading will make them if you supply the metal parts, with whatever hardness of rubber you want. I haven't tried them myself, but a Frazer-Nash restorer was having some made a few years ago when I was in their shop - and he recommended them.
Regards, John
Most, but not all, Robush Silentblocs have a Shore Hardness of 60, akin to the hardness of tyre rubber (assuming it is on the Shore A scale). This does seem to accord with my technical test ..... stick in your thumb nail!!! The problem is that I have no knowledge of the original specification!! Is anyone able to shed any light on the original spec.?
Andy