Rarity (and desirability) of a slant six/ factory 4 speed?

Yeah, the drop is bigger, but if you rev the motor to six grand, you're talking about a drop to just under four grand - right at peak torque for a mildly modded 340 (and actually, I've always shifted my 340 out of low gear at 6500). A slant six that revs to, say 4500, will drop to just under three grand on the 1-2 shift. Again, solidly in such a mild six's torque curve.

Both your Challenger's rear axle ratios are significantly lower (higher numerically) than 3.23. So a low first gear is less necessary. Plus, the supercharged Hellcat has scads of torque to help it off the line. Comparing to old school stuff, a 440 gets off the line well with 2.66 or even 2.47 low gears, especially with the factory Dana 3.54 rear gears. but it will handle even 3.23 gears quite well. In the late seventies, my Dad had a 62 Dodge with a 440 and 4-speed with 2.66 low gear and 3.23 rear gears and it had so much more torque than my 340 that it was all I could do to stay with him off the line even with my 3.09 low gear. But even with the "big" drop when I hit second gear, I could run even with him from there on up.

So, whether or not a 3.09 low gear would be desirable depends on your motor, your car's weight, and the rear gears. All I'm saying is that if you're limited to four speeds, have a small block or slant six, and have a street rear gear, then a 3.09 low gear is much preferable to a 2.66, and I base that on my own experience.