I am planning on doing something like that with my bike once my kit comes in this week. I still got a left over muffler from my first motor. But I plan on mounting mine at about a 45 degree upsweep in back.On my bike that I pull my daughter on I also ran the pipe all the to the back of the bike with dual mufflers exiting really low to keep any fumes out of the trailer. This gave the engine a much quieter and deeper note but like above welding is required.
This pic is before paint but you'll get the idea---- build thread in stretch cruisers under "my first stretch"