Dart 72 vs 73 susp dilemma

It definitely sounds like your 72 has been swapped over to 73-76 front suspension. No need to order both early and late parts. Your car probably had 4 wheel drum brakes when new as they were standard and yes you could order discs on a 6 cylinder car. I personally reuse ball joints, tie rod ends, etc if they are good and the seals are not torn. Original Mopar parts were good, better than the typical Chinese crap. Definitely plan on replacing the Lower Control Arm bushings as most times they are deteriorated originals and have not been touched. Pictures would help a lot! I put a 73 K in my 68 with brakes etc. because I thought at the time I'd be putting a big block in it with spool mounts.

No need to convert back to 4 piston Kelsey Hayes brakes. Going back to SBP would require different wheels.

Whichever brakes were on it when new, your 1972 had the small bolt pattern. That's 5 bolts on a 4" diameter bolt circle and 7/16" studs. 1973 and newer disc brakes went to the more common 5 on 4.5" diameter with 1/2" studs. You should check the rear bolt pattern while you are at it. Could still be SBP (small bolt pattern). Wheel selection is not as good with SBP.