Not sure of track width of the chrysler 300, but the backspacing seem about right from what I've researched for myself. I've read anywhere from 4.5 to 6" back spacing with 8" wide tires. 245 seems to be the norm for the truck, but I wonder if your sales person took into account the front clip. I haven't pulled the trigger and ordered yet, but I definitely want 20" steelies. Your truck looks sweet, so hopefully you can figure it out.
I actually wish there was a way to test some sizes out on these rides before dropping all that money only to find the wheels don't fit. Maybe some measurements like this beforehand would help if you do decide to re-order:
http://www.superchevy.com/how-to/whe...-size-matters/