I agree, but the 340 and 318 exhaust manifold sets are not equal.
The 340 exhaust manifolds are larger than the ones used on the 318. IIRC, the drivers' side manifold exits at a different angle on the end. For certain, the stock 318 passenger side manifold exits straight down at the center of the manifold, the 340 exits at approximately a 45ยบ angle not far from the oil filter.
While the 340 exhaust manifolds would be the way to go, when I've seen them available used, they have been pricey. It may be better to go the TTi route, especially if a more powerful LA motor is in the plans for this car.