What years rear sway bar will fit a '73 Charger? Is there a diameter difference between a 340, 400 or 440 equipped car? I never see the rear bar for sale, only the front. Are there any aftermarket bars being produced?

The bars themselves will swap out between 71-74...The differences I've seen is there's a "dip" in the center to clear the bigger rears {8 3/4,dana,9 1/4} but some have shown the straight bar as original for the 8 3/4...What you do need for sure is the 73-up axle mounting bracket for the isolated rear set-up...You can get them off a road runner,charger, cordoba, magnum etc...Here's a pic of the typical B body bar...

Here's a pic of the isolated rear bar bracket in place between the spring and rear housing...The rear b body bars are still in the boneyards around here for the isolated set-ups...Mabe there's some in your area also..There's aftermarket bars out there but not like the original set-up...And not cheaper than originals either..

MRHWS,

FYI,
There is a Cordoba at the U-Pull-It on Normandy that has a rear bar on it.
If you don't get it, I will.

It is near the back of the Mopar section.