Stock Exhaust Manifold Question

I,v been trying to locate stock exhaust manifolds for my 70 Duster....Motor is a mildly warmed up 318 and I don,t want the hassle of headers, mainly because of the heat soak troubles with the starter and on the left side,the tie rod passes through header pipes. I have a right side {Passenger} manifold that is casting #3418623...fits a 340 motor from 1971-1973.Left side I have 2 manifolds, 1st one is casting #3614395....fits a 318-1972-1973 Dart.2nd one is casting #2951123..fits a 318-1968-1969 Dart.The dilemma I have and this may sound like a dumb question is.....the right side 340 manifold is 1 and 7/8 inch inside diameter at the exhaust exit.The left side manifolds..both of them are 1 and 5/8 inch inside diameter at the exhaust exit.With a 1/4 inch difference exit diameters,would this adversely effect performance of the motor????? in addition to making exhaust pipes bolt up to the flanges on the manifold.the bolt centers on the 2 manifolds from side to side are 1/4 inch different also.. 3 and 3/4 for the right side versus 3 and 1/2 for the left side.Also, the left side manifolds "Swoop" up a bit before going back down....Information I have is I need either manifold casting # 2863553 for a 340 motor or manifold casting #2843953 for a 318 Dart...I,m thinking I need 2 matched pairs of manifolds with equal exhaust diameter exits...

I would be best to have a matching pair , look for a set of 73-74 340/360 A body manifolds and sell the right side 340 , it might almost pay for the later manifolds.

Also not all small block headers capture the steering linkage , it's mostly the cheap ones that do.

Consider the magnum manifolds I'm not sure they will fit but they are very efficient

you mentioned headers so I'm assuming you want more out the eng than just a low perf DD. I'd bite the bullet & get a set of 68-70 340 HP manifolds & they have a ~2&5/16" outlet(s) but they are pricey & might run you $400. The '92/early 93 mag manifolds also have a ~2&5/16" outlets and are a very good choice & will run you $100 + the ride. the later magnum manifolds have a 1&7/8" outlet the same as low po ones & are to be avoided. The very BEST iron mani setup is a 68-70 340 pass manifold and an early mag manifold on the Dr side (it'd be pricey cuz you using half of two different (pricey) sets). The absolute best would be a 68-70 340 HP passenger manifold on BOTH sides. On the DR side the outlet would be pointing forward a bit but its doable or a person MIGHT be able to cut the mani in two & reweld it to position the outlet rearward like the pass one is. As you know ex is the most restricted part of the air pump (the eng) & the 1&7/8 outlets ain't gonna cut it & you know sooner or later your gonna swap in a 360 stroked to 408, so to be proactive...

Yea,that was what I was thinking... I,ll try to find a "Mate" to either one or the other......or else find a "Matching Pair"....

I "Hear" what your saying.....Eventually I would like to get my " Paws" on a 340 and drop it in the Duster.............The "Search" is on....

eBay (pricey) and local CL's are good sources