Might we forget about "muscle cars" for a while and look at a few other things?
Looking at the 383 4bbl Chryslers in, say, 1967 (of which I own one, which was unmolested when I got it in 1981), the ONLY dual exhausts were on the 440HP option cars and maybe the station wagon 440 4bbls. YET . . . the same engine in a Dodge or Plymouth would have had a factory dual exhaust. ALL with the same 325 horsepower factory power rating. Back when engine power ratings were with no under car exhaust system attached, but an exhaust system for the dyno cell itself instead.
The 1965 Chrysler 300 had a 383 4bbl as standard equipment, but as it still had the cam specs from or very similar to the original B 350 2x4bbl from the 1958 Plymouth Fury (252/252), it had 315 horsepower and a factory single exhaust. Power increased with the later "4bbl cam" of 256/260 in '66.
So, my theory is that the same 4bbl B-engine in a Chrysler would have had a single exhaust prior to the 1968 model year, but a similar Plymouth or Dodge might have had a factory dual exhaust as THEY might be more prone to be ordered as an upgrade engine for police vehicles. Yet teh RB 4bbl HP engines would have had factory duals as a matter of course in any C-body. Limiting the discussion to C Bodies only, as similar B-bodies might have had dual exhausts anyway.
In some of those earlier 4bbl engines, Chrysler had what I term "incognito open element" air cleaners. Similar in format to the 1968 Road Runner 383 air cleaners. With about an inch or more of open air space between the outer diameter of the air cleaner top and the air cleaner's base, while many times using the same air cleaner element. Same as taking a '68 RR 383 air cleaner base and using it under a normal snorkel top on a 1968 carb (larger air cleaner mounting ring).
There MIGHT have been some dual snorkel factory air cleaners before this, but it came into prominence on the 440HP GTX motor in 1967. As I recall. Along with the factory HP exhaust manifolds. Although a really nice set of HP manifolds appeared in the earlier 1960s on the 413 drag race cars (but would also fit 383s), although it was reputed that those particular exhaust manifolds would interfere with the power brake boosters' placement. Obviously a hint of what was to come later?
MUCH of what came after the 1965 model year kind of tended to be accelerated by the appearance of the original "Street HEMI" B-bodies. Especially in the area of mufflers. Prior to that, the "hot rod" muffler was the Corvair turbo muffler (which is probably where the term "turbo muffler" came from) with its round case shape. The Street Hemi muffler looked like a normal muffler, but with bigger inlet/outlet pipes, a shorter case length, and very low restriction and general OEM quietness. Before that, the top RB engine option was the 426W wedge engine, with only 5 more horsepower than the similar 413HP engine. Performance figures were lackluster, at best, by comparison. As it was, at that time, the "ultimate" RB engine for the masses, many put it on hallowed ground, by observation. Especially with a 4-speed.
It was always a mystery why my calculations indicated (from magazine road test data covering MANY years), why the 413s seemed "weak" for their power rating, as to how much power was at the rear wheels to move the car. In the middle 1990s, the dealership-sponser of our Mopar club had accumulated a warehouse full of obsolete parts from other dealers, to re-sale for them at reduced prices. In that bunch of parts, I found a '65 B/RB exhaust manifold. I was amazed at how small the exit hole was! Not much bigger than that of a Chevy 350 exhaust manifold, in diameter on a purely "log" manifold. "No wonder!", I thought. Newer designs in '66 with the 440 motor, which were still "log" manifolds, but with larger cross-sections and such. That's why a 440 and 413 had such different actual "on-the-road" performance figures from the magazines, obviously.
1965 was the FIRST model year of the new C-body platform with a WIDE engine compartment, but as the 440 was then-known to be coming in '66, no effort to change the 413 exhaust manifolds to something better. Especially as to do so could mean that the upcoming 440 might not receive the accolades it was due when it appeared. So the orig 440 had 350 horsepower ratings, right between the 340 and 360 horsepower ratings of the single and dual exhaust 413s, respectively. With the 440TNT having the new 256/260 "4bbl cam", later aka "Standard Cam", with a dual snorkel air cleaner and dual exhausts, at 365 horsepower (a bit less in the '66 C-body wagons?)
Chrysler Corp was in the "muscle car" business long before GM or Ford. Back when "muscle" meant stronger and more durable. Which is why Chrysler Corp produced the premier taxi cabs and later police vehicles. When Plymouth got its first modern V-8 in '55, its prior taxi reputation for durability was elevated even more. Interesting thing is that although the Flathead Ford was allegedly the "hot performer", the flathead 6-cyl in Plymouths was dang close to the Ford's power output, as was the Chevy 6-cyl . . . in OEM factory configuration.
Remember, too, that in those prior times, the "luxury" cars had the biggest engines. Which is why those engines were transplanted into many lesser Fords and other "hot rods" of those eras.
Back to the orig exhasut system issue . . . when I got my '67 Newport 383 4bbl in 1981, I noticed the single exhaust system and everything under it looked as factory as it could be (except for a replacement muffler). I pulled out my '67 Chrysler sales brochure and it mentioned the "383 4bbl single exhaust" engine as optional in Newports, standard on 300s. Still 325 horsepower, as were the dual exhaust 383/325 engines in Plymouth Fury models. In some respects, my Newport was optioned as an incognito 300. 383 4bbl, bucket seat interior (same seat patterns as the 300, but with Newport door and instrument panel trim). Center seat cushion rather than a console.
Enjoy!
CBODY67