I have replaced many of them, all I can say is that it's not an easy task. You will need a hydraulic press, and be very careful about fitting them in an exactly determined position in relationship to both the arm and the outer arm mounting. No workshop manual is clear on this, both regarding early, and late (post-77) inner arm mountings. What's more, you only have one chance, if you fit it wrong you have to cut the silentbloc and start again with a new one.
I am currently testing polyurethane inner arm bushes-installed them on a friend's R4 without telling her too much about them :-D . Results are brilliant, but it's still too early to judge if they are a worthwhile replacement to the rubber ones. Only thing that I can say is that installation is a lot easier!