I am converting to disc brakes on my daughter 67 fury sport III convertible and I matched up the lower steering arms and ball joints..they look the same, the lower control arm and tie rod tapered shafts seem to seat correctly. What is the difference between the two. I noticed all the parts houses call out two different ones (if the disc one is available) and much more expensive?? Help please!! What is the issue?

you are confusing two different things.
68 and back disc brakes used a differnt ball joint than the drum of those years.

69 and up disc and drum use the same lbj, and the same lbj as the drum 68 and back.

so if you have the 69 and up disc set up, then in fact your 67 drum lbj's are the same.