Looking at this engineering architecture, it occured to me this is in fact half of a 60 deg. V12.
Did Chrysler derive this from an existing V12 or plan on creating a V12 from slant 6 hardware?
My recollection (bearing in mind that it was a considerable number of shekels ago) is that the design was a response to the tendency toward sloping hoods that arose in the early '60s (see here for an illustration). If the engineers hadn’t slanted the block they might have had to revert to a flathead engine.
Whatever the impetus, the result was (IMHO) one of the toughest powerplents ever to be dropped into a vehicle. During my days as an Impoverished Student™ I inherited a castoff '64 Dart that had accumulated over 200,000 miles — and while the car itself was a wreck, the engine ran like it had just come off the assembly line. I concluded after a while that the only way to kill it would be with a bazooka.
(ETA: I just remembered that another advantage was supposedly a longer intake manifold. Why that would be advantageous I have no idea, but that was one rationale.)
All I6 engines are half of a V12? 
Are you just referring to the slanted mounting? That’s not uncommon for all manner of inline engines. All modern BMW I6s are mounted the same way, for example.
I’ve been told by a long-time Mopar ‘expert mechanic’ that it helped the designers get the intake manifold/airflow better situated to ‘ram’ the air into cylinders with fewer restrictive turns. And having the air-cleaner/intake offset reduced hood-height. Maybe a water-pump (or other accessory?) issue - been awhile since I spoke with the guy. Knew his Mopar stuff though.
Well, reading the link after typing this out shows that first paragraph clearly saying ~same thing I’m trying to say (yeah!). Those slant-6’s are great engines. I beat the heck outta a Dodge truck w/ Slant-6 (my first vehicle I had) back around '80. Never had an engine prob at all, and I wasn’t known for meticulous maintenance then, either. Plenty of folks nowadays still pull them out of junkers to use/sell later (~collectible, or were 5-10 years ago).
I thought this I6 with the cylinder bank canted 30 deg. from vertical was 1/2 of a 60 deg. v12.
Missed edit window, so copy/paste here - ETA: don’t think a -12 is gonna use anywhere near the same accessory eqpt or manifolds, so hard to base an engine on doubling of one ‘side’ (valve/crank stuff as well). Lots of redesign (balance and strength) changes when needing to push twice the valvetrain with a cam for one thing. Plus, the exhaust-side will need to be radically changed - can’t have intake/exhaust in the ‘valley’ with V’s for the most part (not with any efficiency, that is)
ETA ETA note how the slant-6 has exhaust and intake flow from same side of block? That is a big limit with respect to OP, imho. I want that engine!!!
The straight-6 is a much-better balanced engine, too. The BMW ones are really smooth compared to a 4, V6 or V8.
I’ve driven cars with Slant 6 powerplants before. They’re not very smooth, and not very powerful, and the cars they powered were some of the worst pieces of shiat Chrysler ever manufactured. Still, they’re nearly bulletproof.
My truck easily out-pulled (boat and car-trailers/stumps, etc) many other trucks with bigger engines (not really big -8’s of course). I had lots of torque for the displacement that was good enough to easily climb steep ‘cliffs’/off-road stuff without worry about revving up into ‘horsepower’ range of powerband before attempt, just chugged right up without ever fuss. Admittedly lower horsey amounts, but great torque for pulling stuff overall. There could be some jerkiness, yes, at low-rpms but nice and smooth once into mid-range of powerband (ime, ymmv if something was off-balance). Second-gear starts were almost too easy even just dumping the clutch for the most part. OD was ~mandatory for highway driving as well unless you liked pushing rpm’s (which could make it ‘buzz’ a bit into the frame/steering, not ever meant as a high-rpm engine basically, iirc)
Perfect 1st vehicle/powerplant (Thanks, Dad - RIP! Good call on that truck).
Chargers (et al) are considered Chrysler POS’s?! To each their own, respectably. I know of plenty of 2nd/3rd-gen Chargers still with OEM /6’s that ride rather smooth x-country (for meets) even on original internals. No biggie 
I just loved highway driving between Houston/Galveston-Keemah areas and Austin several times a week for a few years in that /6 truck; power -v- mpg was simply outstanding, imho, even for a teen with a ‘learners permit’ when first given it, LOL.
I inherited a Dodge pickup from my grandfather and drove it for 10 years. My erstwhile girlfriend had a Dart. I would not be at all surprised if both those vehicles were still tooling around somewhere, skeletonized from rust but otherwise unstoppable.
My first car was 73 Dodge Dart Sport purchased in approx 1981. Engine kept going and going while the body and chassis disintegrated around it. I patched the rusting lower side panels with roofing aluminum and pop rivets to pass inspection.
Work I was able to do myself on that car.
Replaced / changed - oil - fluids - tires - ball joints - brakes - spark plug wires - re-assembled and re-placed distributor after I broke it
IIRC a mechanic told me once that the reason those engines lasted so long was that they it had a long stroke and extraordinarily low RPMS.
I don’t know what they intended to do, but the straight six was popular at the time and there don’t seem to be any Chryslers with V-12s so it seems likely just they designed it from the I6 for the reasons above and had no plans for a V-12.
I’ve never driven a slant six but my Jeep has the inline six and I love them. I dread the day I have to trade it in for a newer V-6 Jeep.