440 under car headers need advice.

That will be a challenging project. 1-3/4" B-body headers (Hedman 78030) fit quite well in a low-deck A-body with manual steering. There are pages with pictures on BigBlockDart.com. I put them in mine and had very little denting to do. Note that the collectors do not line up with the torsion bar crossmember notches, so I had to fabricate two short dog-leg sections (I used 18 degree 3" mandrel bends).
Unfortunately the RB is significantly taller and wider with the 3/4" increased deck height. That results in a lot more collisions with immovable objects.
There may be other brands of B-body headers (and with larger primaries) that can be made to fit with a 440, but I haven't come across any posts that say it's easy...