I want to know if 318 exhaust manifolds are the same as 73 duster 340 exhaust manifol

Most local exhaust shops will make you custom head pipes for about $100.
Might be a bit more if you need new ball flanges.

Does anyone know if the 73-74 318 Roadrunners had the 340 manifolds?
I know they had dual exhaust, and were rated 170HP (vs the "normal" 318 w/single exhaust at 150).