That is an interesting question.
Depends what condition it's in.
I currently have a 67 belvedere 1 that was a slant six car originally but the slant was long gone before I got it and the car needs a resto.
I am not going to hunt down that stuff so it's getting a 383. If the slant stuff would have been there . ... maybe
It's hard to imagine a 68 charger with a slant. I have a 68 charger currently that is a 383 car and I had one before that had a 440. So i can't imagine a six in one when I'm so used to seeing them in big block form.
In general, I feel slants are more for A bodies. When you consider the valiant was originally designed around the slant and not the other way around, it just feels natural to put a slant in a A body. other car lines...... not so much .... just my .02