I installed full length headers on my 72 Dart I had a few years back, and it was a tight fit, for sure. As I recall, I had to loosen the steering box some, and really work with it on the driver side to get them in there. Took awhile, and it was aggravating, but they will go in.
One thing I don't like about the long headers is the routing of the drag link through the pipes on the passenger side, which equates to road rash starting with the first speed bump (been there, done that, won't do it again). The shorty headers fixes that problem, and the only company that I know that does not route that way and puts out the most superior set of headers to all (not some, all) the competition is TTI. You will pay for them, but the owner is a Mopar man and that's all he does, headers and exhaust systems for Mopars, period.