Thanks for giving me the kick in the head that I needed... ^^^^
To the OP:
Here is the exhaust in my son's 71 Valiant 4 door that came in it when he got it. The engine is not in it, but I have tried to give a top and side view so you can see how it's routed. Maybe without the engine in the way makes it easier to see....
This was a 71 318 with stock exhaust manifolds. I think they should be close to the 68 318 manifolds, or maybe use a set of 71's if they are different...
Hopefully these can help. I'm running out of bones to throw you....