Pulling Engine/Trans over the winter....Header Question??

Not trying to argue, but I’m wondering why they wouldn’t fit a Dart?
Dart wheelbase is 3” longer, wouldn’t think that would affect anything?
I have the TTI 1 7/8” - 3 1/2” and when following the instructions, the went in very easy and fit great.
It's not in the front end.
1970-72 dart & demon have the same front sheet metal, frame rails, k member & firewall.
Same