'67 Dart with '68 idler arm?

I sort of lost track here but your diagram is not much help

The "simple" answer which leads to the next "question" or vice versa is to determine if the original bracket has been reamed out for the bolt presently in there AKA what size is the 67 stud.

THAT is going to determine whether you can even use a 67 arm at all, which has a tapered stud on both ends.

AFTER you determine the above then you can plot a plan of action, namely

If you or find someone to fab / repair what is there to "re" accept the 67 stud

Fab a 68 / later style bracket onto your 67 K member

Or look for a later K. You can use any K, 67--72, or 73 later, for an A body and whatever engine AKA slant or V8 must match

What will change in 72--73 is from pillow mounts to the later (and better) spool mounts, in which case you'd need to change your engine brackets and mounts, which is a fairly minor detail

I've got a 67 K which is out, if you need to I can pull the idler off it (which is worn clear out) and measure the hole and stud for you.