The std rear silencer wont allow you to run the piggy back as it will foul the body of the silencer when the shock compresses.
You need to modify the silencer or buy a new one for the job.
John I have given you the info on how to do this ~ a new silencer to do this is about £60 (made by Fresco, but there are other manufacturers who can help with this part, Jemco is another, all the ones I know of have been comissioned by people in the UK to do this job and allow you to fit piggy back shocks)
The Fresco silencer has two types. 1 bolts to the original Honda front pipe and the other to bolt to the replacement Fresco front pipe (which from memory is about £200)
The comment about the top mount is also correct, as the top of the piggy back will bind with the mounting when the shock compresses, to it needs to be releaved to give you clearence. (most people cut the whole cup off)
Decisions decisions....
