I'm really confused as to why you're coming up so short. It's not as if there's a lot of variance in the distances we're talking about here. ie, crossmember is always in the same place in relation to the rear end, etc. A Challenger is a Challenger in this respect.
Everything between the front and rear cables was missing from my car when I got it, so I couldn't put them side-by-side, but still... the new setup went in with plenty of slack to spare.
Not to insult you with an obvious question, but your pedal isn't depressed, is it? Lol.