'67 Dart with '68 idler arm?

This doesn't have to be so complicated. All you would need to do is have the correct attachment for the 68 and later idler arm welded onto your K member. Its just a piece of steel and is identical no matter what other steering components you have or what year model K it came from 68 through 76. This has been done several times by others who couldn't justify a inflated price for a replacement 67 correct part of less than optimal design.
I don't think the aftermarket offers the piece of K member needed but I'll bet a member here has a bad K member with a good clip on it.