You're now in an area that there are no standards or probably even best practices. Each engine will only accept certain pipes and how they fit to a frame etc will differ a lot. Add to that the things you want out of them beyond the added performance... ie the looks etc.
I don't have cars or trucks etc... but the concept is the same for any vehicle. Just dry fit it first... you'll want to take the body off in your case and then see where things are best to route everything. Once you have an idea of where to best mount it so you aren't interfering with anything, or interfering the least... Then decide how best to point your exhaust itself. I'd point it where the exhaust leaves the least residue to clean up after a run. I'd think out the back would achieve that the best or perhaps out the top somewhere. I'm not sure I've seen sparks coming out of an exhaust if the engine is tuned properly, so I'm uncertain if that is even an issue or not.
From the looks of it it will mount pointing down or out the side after cutting the body I just don't know if there's some kind of spark arrester or if that's even a problem it's a traxxas aluminum tuned pipe