Will 70 Dart V8 K-member fit 65 Barracud?

I see.
Being that old I would confirm all of the welds have no cracks.
My 75 Swinger had 9 cracks after cleaning all the crud off. Hence the reason I had my K member cracks welded, plus the complete perimeter.

Totally agree. I reinforce all of my K members. My ‘74 K had cracks at the steering box mount that was allowing the steering box to walk back and forth by a good 1/4” to 1/2” when the wheel was turned.

I go over all of the factory welds, weld all of the seams (the entire perimeter on the 67+ K’s), and gusset the steering box mount and LCA pin tubes. I also install a skid plate on the bottom to further gusset the K frame as a whole.