What systems are compatible?

I was looking at a used exhaust system for a duster/dart sport and was wondering if a full dual exhaust system minus the full length headers and flanges for let's say a 318 duster be easily adapted to fit a longer 73-75 Dart hardtop coup/sedan with a 318.
Is it as simple as extending the length in one area or is there going to be other fitment issues?
Also, would it be a waste of time to modify it further?

If I could save myself a bit of money on a used exhaust system, that would be awesome, but I figured someone with A body experience here might know.

Anyway, thanks