you have to cut your original pipe just a little before the weld on the older K1/K2 bikes. On the below pic you can just about make out the cut just below the footpegs before teh chrome of the end can starts.
If your buying the can from art then you can also get a link pipe which allows you to reconnect your old pipe with no hassle, you can also get a high level link pipe which brings the can up higher.
For cutting your old pipe most use an angle grinder altho some have tried with a hacksaw.
Anyway make sure you put a block of wood of something behind the old pipe where your cutting so you dont end up cutting into the swingarm.
