You can actually use all the 73-87 lines,... in the same lay-out as they were originally installed on the 73-87 frame.
You need to determine which "width" rotors you have before you order drop spindles.
ECE is a great source for 67-72 parts,... but you might be able to get a better deal on 73-87 spindles from Summit Racing.
I would also look at
www.PerformanceOnLine.com
They sell them for $228.
Plus they are a Supporter of this site.