Shag,
You question was---Will the 66 V8 Drums, Hardware, Bearing, Etc FIT on the 68 Drum Brake Spindle.
Every Piece!
Take off one, put on other, just that easy.
Everything is exactly the same on those C5DA/C4ZA 66 Spindles as the C6OA Spindles on the 68--EXCEPT the Steering arm length, which is 1/2" Longer on the 68 Spindles, AND the Tie Rod hole taper.
Now a little advice, the chances of getting good free operating cylinders, good shoes, hardware not rust bad, is not on your side.
Robbing old rust parts for a brake job will usually end up costing more in the long run than buying new from the onset.
One more Fact here with either the 66 V8 Spindles, or the 68 6 Cylinder spindles, you can install Factory Style K/H 4 Piston Disc Brakes on either as Ford Did on the assembly line on all 65 66 67 Mustangs, Fairlanes, Comets with Disc Brakes---Even the 428 Shebly Cars used these.
Dan
www.chockostangclassicmustang.com