Will 340 exhaust system fit 318 (1971 Duster)

Swinger 340 said
I have match them up before and I found an 1/8 inch gap at the bottom of the manifold to the exhaust port. The "J" head ports are much the same as the "X" ports. The 273 and 318 both have the smaller exhaust port as well as the smaller manifold to match. Not sure what you mean by "early A-body manifolds". Maybe some early manifolds were larger than the ones I had.

There is only one set of LA exhaust manifolds for an Early A (1964-1966) and they came on 273 engines. Ran "J" heads for years until the gas wasn't good enough to run 10+ compression ratio. Still have the manifolds, but they are on the car now with 273 heads. I never ran "X" heads on the 273, because of the 2.02 intake valves, 1.88 were close enough at +.040.