I also think your car is pretty cool and unusual, but oddballs are the norm with old Chryslers. I had a fairly-loaded '72 Charger Rallye with a tach, A36 Axle Performance package, dual exhaust with bazooka tips... and a 318.
Come sale time, I had nothing more significant than a 318 Charger Rallye. Rare does not equal desirable. Ask any 2nd-gen Slant Six Charger owner or someone with a 400 two-barrel GTO (yes, they built them). Rarity will
never out-value a factory performance downgrade. That '72 Charger would've been much more valuable had I swapped in a 340, 360, or any big block.
There are two possibilities here: If your car was ordered--most were not--the buyer could swing the $85 required to get a tach, but not the $192.85 required for the extra gear (or he already had a line on a used 4-speed). More likely, it was a Sales Bank car (Y16) and the 4-speed or automatic was omitted to keep the sticker low, leaving more margin for sales-desk haggling.
There's a 1-of-32, full-boogie restored 1974 'Cuda 360 about 40 miles up the road. I looked at that car way back in '90 or '91 and thought, "Shame about the 3-speed." I still think that, I'm sad to say, and I think most others do too. The owner has the OE 3-speed handle and knob poking up from a factory boot. Beneath that there's an adapter to fit the 3-bolt Inland handle to a Hurst shifter, which is connected to an A833. Personally, I'd have all the original parts set aside (for a future owner) in favor of that 4-speed with a Pistol Grip... which I'd leave in at sale time to increase the car's value.
Enjoy the car for what it is, and what you like about it. A 4-speed is a bolt-in, so there's no shame in swapping if that's what you prefer. And while it's fun to tell someone your car is
estimated to be one of 289/3-speed 340 cars, don't expect anyone to stagger backward while whipping out their wallet over it. Remember, you're unique... just like everyone else.