i have a 86 k5 ... will the pipe from a 250 fit?
With enough time and effort you might be able to make it fit but I think you would be far better off trying to find the correct pipe for your bike.
The K5 pipe takes a few twists and turns that the 250 pipes did not.
According to the book, only the 85 KX500 pipe will fit.
However, I suspect that a few others years would be much easier to adapt over.
I know someone on this forum knows for sure....