I have Stock 68 318 manifolds. I have no Idea if there is a variation to them. The exhaust guy I think knows what he is doing....He has been doing exhaust's for me for the last 15 years. (all custom) He was concerned about how tight the bends would have to be, and clearance. There is only like 1 3/4" y pipe on the car now, not much more room in diameter in prox. to the tie rod.
I thought there would be more room under the car.
Did the 318 have an option for dual exhaust in 68?